Troubleshooting portables that won't turn on or start up
Troubleshooting: My computer won't turn on
Other things to try.
See if you can open the disc drive. Depress the mouse pad at startup. If you get it open, insert a system disc & see if it will start from the disc. Depress the C key at startup.
Will it boot in Safe Mode? See
What is Safe Boot, Safe Mode? (Mac OS X)
Takes a while to run, but it usually "fixes" problems.
Resolve startup issues and perform disk maintenance with Disk Utility and fsck