Usually when there are two accidentals like that, for example a natural and a sharp, it means that earlier that note was
double-sharped (this symbol looks like an "x"), and the composer is cancelling just one of those sharps.
So in this case you would play a C#.
If the composer wanted to cancel both sharps, you'd see two natural signs in a row, and you'd play a C natural.
Are you sure there isn't a double-sharp altering that C, earlier on? I can't think of any other situation where you'd have the natural and sharp right next to each other like that. Maybe if the note was altered in some other way earlier...
-Rob