pull the battery, disconnect the Pram battery, hold the reset button in for about 20 seconds (reset button is next to Modem plug in) plug in the AC adaptor and push in the start button. It should power up. If does not.

Do this. hold down the Option/Apple keys hit the power button and then hold down the p/r keys. hold them down until you hear the machine bong 3 times. Release them after the third bong and the machine should boot up.

A dead Pram battery will cause most of the issues you are experiancing. You can leave it disconnected and as long as you have a power source the machine will boot up. Recommended fixs are new or good used main battery or replace the Pram battery