Try starting up after each of the following (only do what you feel comfortable with)
Verify the power outlet is good – try a different one on a different circuit and make sure nothing else is plugged in (the power coming from your outlet could be wrong/bad and you may want to have an electrician check it out)
Replace the power cord
Make sure the voltage switch on the back of the power supply is set to the correct voltage
Disconnect any external devices (monitor, keyboard, mouse, firewire drives, scanner, printer, etc)
Remove internal cards
Disconnect internal hard drive/s (it won’t completely startup, but will get far enough to see if the HD was the problem)
Take if back to an Apple authorized repair shop ( you may have a bad power supply or logic board. Getting two bad ones in a row is extremely unlikely though, I’ve never had one and I’ve bought a lot of Mac’s. I would suspect your homes power if that is the case).
Good Luck,
Tony!