How does an automatic watch work?
An automatic watch is a mechanical watch that winds itself through movement.
When you wear the watch on your wrist, a rotor moves inside it. This rotor turns through your arm movements and transfers energy to the barrel. There, the energy is stored in a mainspring.
This stored energy powers the movement. How long the watch keeps running when it's not being worn is called the power reserve.
For an automatic watch to run reliably, it should be worn regularly. If it's left unworn for a longer period, the power reserve may run out and the watch will stop. This is normal and not a fault.
In this case, you can wind the watch manually using the crown and then wear it again.