If your watch is mechanical, it may need winding. A mechanical watch will usually need around 40 rotations of the crown from completely unwound — a manual watch will resist when full, an automatic will go on winding forever. The crown on water resistant watches may need unscrewing before you begin. Automatic watches can run out of power while being worn if your movements aren't sufficient to keep the mainspring fully wound. If your watch has a quartz movement, it might need a change of battery.