I hit the on/of button next to the power chord cable when my computer Was running. Now it won't boot

Might be that your processor powered off during a disk write and your hard drive data is corrupted.. Can you get into BIOS?

A second thought. Did you perhaps immediately flick the power switch back to "on"? THat might have caused some surge that took out stuff. Anser the first question and we'll go from there.

Turn PSU main switch off, remove CMOS battery, WAIT 15mins.

It'll be okay, just do a power clear all the way.

The BIOS reset process that was recommended above was incomplete. Here's the full sequence.
1. Turn OFF the power switch on the back of your PSU next to the cord. Open your case. Find the BIOS battery (silver, about the size of a quarter) AND a set of 3 pins with a jumper on two of them nearby. The label should say BIOS Reset.
2. Remove the battery. Move the jumper from one pin set to the other for at least 15 sec. Then move the jumper back to its "normal" position. Replace the battery. Close up your case.
3. If you are using on-board video and NOT an added graphics card, skip the next step (#4).
4. NOW we need to check how your system sends video out to your monitor. IF you have a mobo that included on-board video PLUS an added graphics card, it is possible that resetting the BIOS as above will set it back to the default position of sending video out ONLY from the chip on the mobo. This means that your graphics card will NOT be in use. So, you have to change where the cable to your monitor is plugged in - plug it into the video output on the panel on the back of your computer. Next, make sure your monitor is set for that feed. IF the new connector is the same type as you were using before (e.g., both are HDMI), you don't need to worry. BUT if it is a different type (e.g. was HDMI, now is DVI) you will need to tell your monitor what input to expect.
5. Turn on the power at the switch on the back. Now, you want to boot directly into BIOS Setup. Do you know what key to press? On many system it is the "Del" key, but some are different. When you do this, watch your screen - we hope it will have at least brief info. Usually at the bottom you'll see a message about what key to press to "Enter Setup". If it's not "Del", make note and try again.
6. Push the front panel On / Off button to start the machine. Immediately press that key to enter BIOS Setup. But don't just push and release - the mobo may not notice that. I always just hold that key down until the monitor shows the opening BIOS Setup screen. IF that does not show ANYTHING on your monitor, check the monitor input setting. If that's OK, MAYBE the system BIOS figured out on its own that you have an added graphics card and set itself to use that. In case that has happened, re-connect your monitor to the graphics card, set the Monitor's input setting to accommodate that, and reboot the machine to try again.
7. By now we would hope that you have been able to get a display on your monitor and enter BIOS Setup. If that is not so, you have a serious hardware problem. But if you have reached BIOS Setup, proceed with the rest.
8. In BIOS Setup, look near the end of the various screens for a place where you can Load some Default Settings, etc. Usually the Optimized Default is best, but plain Default may be fine. Loading these ensures that a complete set of defaults is in place.
9. Now you need to restore any customized settings you had in BIOS before. At minimum there are two that will need attention. (a) IF you have an added Video card you are using INSTEAD of on-board video, make that setting change. (b) go to where you set the Boot Priority Sequence and ensure that the proper sequence of boot devices is set so it can find you OS to load.
10. Make any other BIOS changes you know you need.
11. Remember to SAVE and EXIT so your settings are saved. This should then re-boot you properly.

The BIOS reset process that was recommended above was incomplete. Here's the full sequence.
1. Turn OFF the power switch on the back of your PSU next to the cord. Open your case. Find the BIOS battery (silver, about the size of a quarter) AND a set of 3 pins with a jumper on two of them nearby. The label should say BIOS Reset.
2. Remove the battery. Move the jumper from one pin set to the other for at least 15 sec. Then move the jumper back to its "normal" position. Replace the battery. Close up your case.
3. If you are using on-board video and NOT an added graphics card, skip the next step (#4).
4. NOW we need to check how your system sends video out to your monitor. IF you have a mobo that included on-board video PLUS an added graphics card, it is possible that resetting the BIOS as above will set it back to the default position of sending video out ONLY from the chip on the mobo. This means that your graphics card will NOT be in use. So, you have to change where the cable to your monitor is plugged in - plug it into the video output on the panel on the back of your computer. Next, make sure your monitor is set for that feed. IF the new connector is the same type as you were using before (e.g., both are HDMI), you don't need to worry. BUT if it is a different type (e.g. was HDMI, now is DVI) you will need to tell your monitor what input to expect.
5. Turn on the power at the switch on the back. Now, you want to boot directly into BIOS Setup. Do you know what key to press? On many system it is the "Del" key, but some are different. When you do this, watch your screen - we hope it will have at least brief info. Usually at the bottom you'll see a message about what key to press to "Enter Setup". If it's not "Del", make note and try again.
6. Push the front panel On / Off button to start the machine. Immediately press that key to enter BIOS Setup. But don't just push and release - the mobo may not notice that. I always just hold that key down until the monitor shows the opening BIOS Setup screen. IF that does not show ANYTHING on your monitor, check the monitor input setting. If that's OK, MAYBE the system BIOS figured out on its own that you have an added graphics card and set itself to use that. In case that has happened, re-connect your monitor to the graphics card, set the Monitor's input setting to accommodate that, and reboot the machine to try again.
7. By now we would hope that you have been able to get a display on your monitor and enter BIOS Setup. If that is not so, you have a serious hardware problem. But if you have reached BIOS Setup, proceed with the rest.
8. In BIOS Setup, look near the end of the various screens for a place where you can Load some Default Settings, etc. Usually the Optimized Default is best, but plain Default may be fine. Loading these ensures that a complete set of defaults is in place.
9. Now you need to restore any customized settings you had in BIOS before. At minimum there are two that will need attention. (a) IF you have an added Video card you are using INSTEAD of on-board video, make that setting change. (b) go to where you set the Boot Priority Sequence and ensure that the proper sequence of boot devices is set so it can find you OS to load.
10. Make any other BIOS changes you know you need.
11. Remember to SAVE and EXIT so your settings are saved. This should then re-boot you properly.