This could be a for a few different reasons. First, you need to determine if you have an automatic or a quartz watch. A quartz watch is powered by a quartz crystal that runs off of a battery. If you have a quartz watch, and it stops keeping time, most likely the battery has died. We inspect all watches that leave to ensure they are working but sometimes, unfortunately, the battery life can be short.
If your watch is an automatic, and it stops keeping time, then it needs to be re-wound. Automatic watches are powered off of the movement of the wearer’s wrist. Through normal activity a rotor turns and winds the watch's mainspring. Most automatic watches have up to 36 hours of power reserve. If an automatic watch is not worn for a day or two, it will wind down and need to be wound by hand to get it started again. You can also use a watch-winder for this function.