SWF 601C encoder(?) stuck - HELP! please

I was just starting a job on my compact machine. Had stitched maybe 6 stitches and went to stop it because I needed to manually trim a long thread at the start. I think I accidentally hit 'start' rather than 'stop'. Machine ground to a halt and started flashing the error 100 msg. I can normally clear this by turning the wheel at the back to the 100 postion. However, today, it is completely frozen. It won't turn in either direction. I've tried turning the machine on and off - nothing helps. Have an important job to finish today and don't know what to try next. Would appreciate any advice. Thanks!