Let it boot. It will, eventually. Then, give us a call and we'll talk you through a quick fix for the problem.
Computers are much like cars.
They need regular maintenance. Once in a while, use some canned air and blow the dust out of your power supply fan. Get a floppy drive cleaner and use it every couple of months. Get a CD Drive cleaner and do the same. At least once a year, bring in your system for a maintenance check.