Need help determining what is causing issues on startup

After a series of the power being shut off without notice while my computer was powered on, my computer would no longer power on. I figured it was the power supply initially so i put in one that i know works, and now the computer will power on but will not boot up. when i power on the computer, the monitor doesn't recognize it and goes to sleep. the hard drive it's being booted off of beeps twice and then nothing. My guess is that it's either the mobo or the HDD thats the problem. Any knowledge or advice would be appreciated!
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More about need determining causing issues startup
  1. Start by unplugging the PSU and pulling the CMOS battery. Let it sit for ~30 seconds, then reinstall the battery and power to the PSU, and try to boot. Sometimes just clearing the CMOS can get stubborn machines to boot. If that doesn't work, pull all the RAM but one, and see if that boots. Those are the quick and easy remedies that help weed out issues, if they don't work, reply and I'll help how I can!
  2. ^+1 I'd add that while waiting 30 seconds with the battery out, press the case power on button a few times.

    You need to ensure all power is removed from CMOS so it clears. Pressing the power button helps discharge mobo capacitors that may keep CMOS from clearing.
  3. That helps.

    Unless, of course, you have a silly P965 Gigabyte board that insists on running that little LED on the mainboard until every bit of juice is gone :)

    *Wait, is that why that things wired like that??*
  4. Work through our standard checklist and troubleshooting thread:
    I mean work through, not just read over it. We spent a lot of time on this. It should find most of the problems.

    If not, continue.

    I have tested the following beeps patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different.

    Breadboard - that will help isolate any kind of case problem you might have.

    Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here:

    You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU.

    Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.

    If no beeps:
    At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

    The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

    You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.

    This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.

    If the system beeps:
    If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card. Silence or long single beeps indicate a problem with the memory.

    Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.

    Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.
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