Who is the manufacturer of your computer? I would call their support line if it is under warranty. You will probably need to open the case and remove all pci cards and all but one memory module to start with. Try booting, and if successful add one card or memory module in at a time and again see if the pc boots. If not successful you are probably looking at a motherboard issue. Try different memory modules and try them one at a time and in each memory slot until system boots, if it will boot. Basic hardware troubleshooting steps are required here. What did you change, if anything before the problem began? Did you add a new card or peripheral? If so, remove it and try again. etc etc.
Also make sure the PSU is working as expected.
We had a similar situation a few months ago, and the problem was in the power supply.
(LoneWolfBW) – It also might be a heat issue. We found one of our servers shut itself down without warning, I found that the CPU heatsink/fan had stopped functioning and it was powering down automatically to protect against overheating.
(Koohiisan) – Is your power button messed up? We’ve had a few that were stuck in, so that when you turned it on it immediately sensed that you were holding the button in to turn it back off! Make sure it’s not crooked or sticky!