The most common reason why a hand-wound watch stops is that a screw has loosened, which then blocks the movement. This often happens due to strong vibrations, because you wore the watch e.g. doing sports. To ensure that the manual wind watch runs again, it is opened, the loosened screw carefully removed from the movement and put back in its place. On this occasion, you should also have a water resistance test carried out immediately after closing the watch.
It gets more complicated when dirt and dust block the movement. In order to get the watch up and running again, the watch has to be revised. The entire movement is disassembled into its individual parts, the individual parts cleaned, lubricated and reassembled.
Another reason why a hand-wound watch stops moving has already been mentioned above. It is the broken spring. Due to the tension pressure, the winding spring can tear if it is overtightened despite the mechanical lock or if the spring is older and therefore somewhat porous. Especially with older elevator springs, the maximum tension provided is sufficient for this to happen. For hand-wound watches that are already a few years old, a lot of wariness is therefore required when winding.