What is a power reserve?
The power reserve describes how long a mechanical watch keeps running after being fully wound.
With an automatic watch, energy is stored through movement of the wrist or by manual winding. This energy is held in the mainspring in the barrel and drives the movement.
If you wear your automatic watch regularly, the power reserve keeps getting topped up. If the watch is left lying for a longer period, the stored energy is gradually used up.
Once the power reserve is fully used up, the watch stops. This is normal for mechanical watches and not a fault. You can simply wind the watch again, set the time and then wear it.