could be a larger than normal bean getting stuck... it happens...
adjust the grind: *open the burrs up to a really course setting, *grind a second or two, then return the burrs to normal setting.... that is normally enough to clear the offending bean.
you will have to dump the grinds and start again for your god-shot, tho *;)
if the grinder still doesnt want to move, it could be a stone or piece of metal holding the burrs... and you will need to pull the grinder apart!!