== If you hear no ==
startup chime your machine is failing POST (power on self test). You could be trying to boot an OS X machine into OS 9
Eliminate [http://support.apple.com/kb/TS1411#symp5|thess problems]
OR The CMOS Batteries under the optical drive aren't contacting properly and cuts the power out
'''If this answer was helpful please remember to mark it accepted.'''
Good Luck,
N