Try starting your Mac in Safe Mode.
- Restart your Mac.
- Press and hold down the Shift key immediately after hearing the startup tone. Be sure NOT to press the Shift key before the tone.
- Release the Shift key when you see the Apple logo and the spinning-gear appear.
- When the Login Window appears, log into your account.
If you Mac starts up, then the issue may be a problem with one of the Startup items, Login items, or a 3rd-party kernal extension (if you got a kernal panic screen during startup.)
If your Mac does not start up, use the Startup Manager to see if the startup disk is detected. Note: By default the startup disk is Macintosh HD.
Try running the Apple Hardware Test. This test is included on the Installation CD that came with your Mac.