Tell the group more about your computer, make, MB model #, Windows version your are using, and if you have a backup or image of your system disk. Also if you have a UPS or surge supressor on your system.
Any steps you have done to begin troubleshooting the problem will help the group to better understand what happened and come up with the best steps to repair your system.
A couple first things to check, does the power light come on at boot up, and do you hear the HDD begin to spin up? Then click the <del> (system setup key) to see if you can get to the BIOS system setup. And do you hear any "beep code" sequence from the computer?
I have had this happen and found a solution for my machine - not sure if it can work for you. What I did is a complete cold restart.
On the back of the PSU, switch off the master switch there, or unplug the power cord. Leave the machine completely without power for at least 5 minutes. Then reconnect the cord or turn on that rear switch. Now push the front panel button and see if it turns on again.
Well, one thing that would explain both problems is that your power supply decided it was time to die.
Another thing that could explain it was that the system overheated to the point where the CPU was damaged - though most CPUs these days are smart enough to shut themselves off before anything serious happens.
Does the system power up at all when you turn it on? (In other words, is it completely dead, or is it physically running and you just get no video?)
Anyway ... hard drive is about the last thing I would guess as a possible cause.