Have you checked your DVD/CD drive for a disk? If there is one in it it may prevent you from booting. Same applies to floppy drive or similar peripherals.
Failing that you could check for bad connections (power and data) to any drives or disconnect them completely.
BEFORE you open the PC ensure all power is OFF and the mains plug is out. Touch the chassis of the PC befor any components to ensure you and the PC are common in any voltage.