Power light flashes

On starting up my desktop, it appears to be going to work (fans start spinning). But almost immediately (after 1 -2 seconds)I get a bleep, the fans stop and the machine is left with the power light flashing.

I have to remove the power cable for while to get the machine to work, though I might have to do this two or three times to get the machine to power up properly.

I don’t know how to fix this. Can you help?
TIA for your experiences in this….

B