Try to reinsert the stem into the Eco-Drive watch and pull it out again. If you cannot easily pull it out, and the watch still runs, it is probably fine. The detent (the clutch) that holds in the stem probably just slipped. However, if you can still pull the stem out, the detent is likely broken. If the watch no longer runs it is also likely that some of the gears are also broken, so you should have it examined by a reputable Citizen Eco-Drive repair shop.