In my experience, causes that may stall or hang the Power On Self Test are listed in order of frequency:
- Hard drive has failed
- Peripheral devices (USB, Firewire) malfunction
- CPU model not supported in BIOS
- Ram chips not supported (avoided by pre-purchase research)
In the third case, the solution is to install an older CPU that is supported by the BIOS and flash the motherboard to the most recent version. This process varies depending oh the manufacturer of the board and the version of the motherboard. Consult the user manual or the support web site of the manufacturer for both the instructions and list of supported CPUs.
In the last case, I think that the BIOS would produce an obnoxious beeping sound with regular intervals, such as the case when no RAM chips are installed as well.