Have you checked in BIOS(press DEL at bootup)to see if it is reading your hard drive? You say you have checked your drive connections, but have you got a dodgy cable? Try replacing the one with the drive on it....TT
But just as an update, I couldn't get anything up on screen at all so couldn't check BIOS settings etc. In the end, I removed and replaced the hard drive/graphics card/RAM and this did actually do the trick as the PC did then boot up.
Hasn't happened again since so can only put it down to a dodgy connection somewhere.