Win 7 over Vista worked fine now computer is dead

[asamd1st]

Several weeks ago I installed Win 7 over Vista on a 64-bit HP desktop. Now after a routine reboot atempt machine acts as if it were dead except the green light on the back showing power on. What now? To make maters worse I had the same problem and found an easy solution but now I don't remember what it was.
Help!
Thank you

[Rik from RCE]

If it won't fire up at all then it is very unlikely to be a software fault.
Do you get anything on screen at all?

HP use the cheapest possible components so replacing the PSU (especially if it is a Bestec one) might be worth doing as the green light only means that the 5 volt standby is working, nothing more.

If the computer has had any upgrades like more ram, bigger hard drive, or more powerful graphics card then the PSU may well not be up to the job any more!