Although you may think beside the point .... computer PSUs are a regulated power supply ...... with no load , half load or full load.....the 12V rail will show 12V......unless you have some pretty fancy paper clips and ohmmeter.....you want be able to test the PSU at home!
Follow DMs advice because this is the correct procedure/fault find ..... but don't rule out the PSU
you also need to let us know what the "Beep" code was......long, short, how many and what order!
Aussie Allan