First thing of course is to unplug it and let it sit for a minute or so. If the machine still works but the readout is zero then there may be something wrong with the speed sensing sensor. I could not find where the readout sensor is on the parts diagram. It could have slipped out of position. I believe I've read about that happening. I have a 3520A. I can look and see where mine is but it's probably not in the same position since they added indexing and change the spindle lock position and some other changes on the B series.