I was shooting an engagement today, tried to mount the 70-200 vr onto the D50 and it focused at first, then it wouldn't auto focus. I didn't change any buttons, it was m/a as normal. Camera set to continuos, etc. Any way to test the af-s motor or should I just plan to send it back into Nikon? If it is the af-s motor, any idea on how much they charge to fix it? I need it back before football/soccer season and a wedding in October.