I have an HP desktop about 4 years old, I was using it a few days ago but for some reason now it doesn't turn on. The last time I used it I left it on afterwards and the next day it wouldn't "wake up" from sleep mode. I turned it off using the power button, unplugged everything and reconnected it all. Everything is properly plugged in, all the switches are on.
What can I do?
It's probably the PSU has failed or the ram is faulty, a bit hard to say. Have you done any recent hardware upgrades? A bios update? What OS and what motherboard model and make?
Assuming it is Vista and considering that it is HP, then I would say that your PSU is only about 305 watts and if you have fitted a better video card then the PSU has fried the motherboard, ram, vidoe card, etc by not allowing sufficient power.