What does this error mean for gprof? I'm running gcc -pg.
Hard to tell with so little information. This can happen in particular if you messed up with the arguments you're giving to gprof, as reported here (which was hard to find, because it's only Google's 2nd hit for your question's title).
Edit: so, double check your arguments. If you want us to check them for you, report the command-line you used, and how the various files were generated. As a rule: we can't guess.