Mechanical watches
If the watch supports manual winding, give it an appropriate wind and observe whether it starts and how long it runs. Some older automatics—such as Seiko 7S-series movements—do not hand-wind from the crown.
Quartz watches
A suitable fresh battery is the obvious first test, but dead circuits, corrosion, blocked hands and failed movements can produce the same symptom.
Solar and rechargeable watches
Give the watch a proper charge following manufacturer guidance. A light-powered watch stored in darkness can be deeply discharged without being mechanically broken.
Recent events matter
Drops, water exposure, magnetism, crown damage or a recent battery change can all change the diagnosis.