Have you made any changes in the BIOS cpu or RAM settings recently, or OC'd the cpu via software ? If so boot to BIOS and revert to default settings, and see if that helps.
If you can't even get into BIOS reset the CMOS via the motherboard jumper (power disconnected), or remove the battery for a minute or more before replacing it.
There is also some chance of bad capacitors, or power instability in the psu (fairly common with cheap psu's - they age quicker than better quality psu's with better quality components). You may also want to check the motherboard. They can sometimes get caps that age prematurely. Just look for bulging tops (they should be dead flat) or leaks ( tan powery substance).