'99 Crankshaft Problems (simple question, please help!)

I recently took my car to a mechanic because I was having intermittent starting problems. They in turn fixed the camshaft position sensor, the waterpump and the timing belt. I drove my car away and within 25 miles it completely shut off while I was driving it. This has NEVER happened before. They are now telling me the Crankshaft gear is 'sheered off'. Is it possible that this happened as a result/during them repairing my other issues? Thanks!-j 99 Jetta