I hhave just bought some new parts for my PC a
motherboard
processer
gfx card
power supply

I have put all the parts in and wired them up and pressed power on, but it didnt boot up.

The computer switches on but the is no video output and it sounds like it keeps on restarting with a little beep every few seconds and the disc drives reving up.

I realise help based on that info wont be easy but any ideas?

Thanks

Does the motherboard have an onboard video card as well?

There could be a million reasons for what is happening, so unless you have more info...

Anyway, basically start a diagnostic process. If the board has an onboard video card, switch to that, and remove ALL the other cards. Only have what is absolutely necessary to boot the machine. Also make triple sure that the processor and RAM are firmly attached and in place.