If you're positive it's never been dropped, then it could be something like a screw worked loose. The clearances between the moving parts are tiny, and sometimes a screw protruding a tiny bit can make a difference. If that's the case - if - then it ought to be a cheap repair.
Before sending it off for repair though, remove the rubber focus and zoom grips and check whether there are any screws in the barrel under the grips that need tightening. I honestly can't remember whether there are any on the 70-200 - it's an extremely reliable lens, so we rarely have any reason to investigate its construction. But for some types of lenses, this can be an easy DIY fix to problems with moving parts not moving properly.
