Grey screen can go no further

These MacBook Pros are known to their GPU failure.

If you notice that a progress bar appears at Gray Screen, you will consider to replace the HD or format it and install new OS X on it cause the drive or the drive partition is failing. You can buy a 750GB HD here if you prefer because I found a fast drive with 7200RPM 16MB Cache Memory for you.

If you do not see the progress bar but the Gray Screen only, your GPU failed. You can send it to the repair shop like this. They are known for logic board repair services. This kind of service is cheaper than buying logic board. Or you may consider upgrading Mac in this case.