Hi I have a HP m640n series computer and today i went to turn it on but when i pressed the button nothing happened. I noticed that the green light on the back of the power supply that is usually turned on when it is plugged in was not lit up. I figured that i need a new psu. Does anyone else know anything else that cuold be wrong with my computer?

The power cord can become lose from the computer, on PC's it has to be pushed in really hard. Also confirm the plug in the wall is OK (plus confirm that outlet is working too - ie switchboard relay may be off)

i already checked a few different power outlets in my house and the powerbar its connected to is turned on, i'm also sure that its plugged properly into the computer.

OK

This has worked before

1. Remove the power cord
2. Hold the power button on the PC in for 30 secs (dispersing all voltage)
3. Let go of the button
4. Plug the power cord back in (securely !)
5. Turn on the computer again

If that didn't work, I'd have to agree Power Supply

That didn't work. I also tried the power cord in another pc and it worked. I am probably going to order a replacement psu from HP. Is there somewhere else that I can get a bigger psu that is still compatible with my motherboard?

Yes (and better)
But actually your system HP m640n can you just re-confirm that, or find it on the HP site - support - start detection (to confirm type)

whoops its the HP m7640n media center Pc

http://h10025.www1.hp.com/ewfrf/wc/product?product=3245032&lc=en&cc=us&dlc=en&lang=en&cc=us

One last option !

You will need to open the case (hardware experience required and safety precautions from manual must be understood)
Power cord out !
Remove cover
Re-connect all power cables internally, especially to the motherboard
Also actually smell the power supply, for that electronic burnt out smell
Try again

Otherwise any computer shop can sell you a power supply, I recommend min 450Watt

ok, thanks for the help