Wenn ein Apple Store in der Nähe ist, direkt da hingehen.
Wenn der Start-Sound noch kommt, Recovery mode ausprobieren: Command-R beim Starten, dann mit Disk Utiliy die Platte checken.

Ansonsten vllt. mal SMC reset:

Shut down the computer.
Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

Oder PRAM reset:

Note: To print these instructions, open Help Viewer’s Action pop-up menu (looks like a gear), then choose Print.
Shut down your Mac.
Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on your Mac.
Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears.
Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.

Release the keys.