Nothing to worry about, it can happen.
Be sure your drivers are up to date. This might help minimize the chances of it happening again.
it's very common for the drivers to crash when you are overclocking (it means the overclock failed) Try adding more voltage or turning down the core clock.
If you were running at completely stock speeds, I'm going to guess it was just a one off crash, you got unlucky.
Also it might be worth mentioning it could also be the games fault. if this is the case, all you can do is complain to the developers, and hope that they fix the issue.