will not turn on

First try resetting the SMC by unplugging the machine and external drives. Press and hold the power button for five seconds, then plug it back in and give it a try. Please give us the last three figures of your serial number. Note and lights on the logic board before and after you try to power up. Have you ever changed the PRAM battery? Can you test it with a multimeter? Any symptoms before it quit on you? Was it holding the date and time and printer setting?