If in doubt, the first thing to do is nothing.
Then, google the error message, and read a reasonable number of the hits:
Google[
^]
They will give you suggestions - don't do any of them until you have read a couple and got a good idea of why this might have occurred and what the effects of each suggestion might be - if you do the wrong thing too early, the chances are you will make the likelihood of data recovery lower, not higher, by overwriting your existing data. If you find suggestions that fit what happened prior to the problem being detected, then all the better.