Gray Screen on Boot

Hold down CMD + R as you restart until you get to Mac OS X Utilities. Click Disk Utility > Click the Hard Drive, and then Your Hard Drive's Name if displayed underneath. Click "Verify Disk," and wait until it completes. Click "Repair Disk" if available. Restart.

If you see the same gray screen after reboot, restart while holding CMD + S. You when you see a black and white screen, wait until the command prompt appears, and type: fsck -fy and hit enter. You'll see one of two messages: "OK" or "FILE SYSTEM WAS MODIFIED." If you see the "OK" message, type: reboot, and proceed to the next step if needed. If you see the "FILE SYSTEM. . ." message, press the up arrow and hit enter (to run the fsck -fy command again).

Hopefully after trying the fsck -fy command the first or several times, you got the "OK" message, and either restarted to a usable system, or: Restart again holding CMD + Option + R to enter Recovery mode again. From here, you can try running a repair from Disk Utility again, or reinstall OS X.

Recommended: If you aren't already, plug in an Ethernet cable. Reinstalling OS X over Wi-Fi can take a while.

From Disk Utility, click "Reinstall Mac OS X," and follow the onscreen prompts.

Hope this helped. If you have any questions or get stuck, please let me know. Recovery can be frustrating!