pc problem...

my pc was working fine before it went on standby after 20 mins,when i tried to activate it,the monitor still remained on standby,cuz usually when you move the mouse or press a key,the monitor turns back on,but that time,it didnt turn back on with the light blinking,as if still on stanby,i tried restarting,then an error displayed,cannot find the SYSTEM folder or something,i tried restarting again,this time it didnt boot,tried replacing the memory and the video card,still no changes,the memory and video card of that pc is working fine,tried it on another pc,i tried detaching the hard disk and floppy,cleaning the memory,and video card terminals,its still not booting,could it be the BIOS?
cuz on my first attempt on restarting it still booted,but on the 2nd try it didnt boot anymore.i tried everything i know,but still it doesnt work...help me pls...

 

doesn't sound too good. you could try to remove your battery of your bios so the bios will be reset (just keep it out for 2 min.)
could be your psu is giving up
how old is this pc?