FATAL ERROR: trying to write to readonly database

I am a beginner to Unity and was following the Roll-a-ball tutorial. I was pretty far in my tutorial and all of a sudden I got a box that said "Fatal Error" and the only option given was to quite the program. I was experiencing this problem too when I tried a different tutorial and had to import assets and when it was almost finished importing the same message popped up with the same effect. Now when I try and go back and open either files I am unable to and the "Fatal Error" message pops up. What can I do to assure this wont happen.