Not 100% on this solution, It was years ago, but I still remember it did happen to us when we were working in another company.
I was told it was the belt under the head cover, on older machines this belt I think goes to the encoder. You just have to remove the head sheetmetal cover and look up top of head and you will see it.
Might of either broke or is off the pulleys. I would check it out there first.