How do I fix a switch that doesn't work?

First, check if the bulb needs replacing or if a GFCI outlet has tripped on the same circuit. If the switch itself is faulty, turn off power at the breaker, remove the switch plate, and use a voltage tester to confirm power is off. Replace the switch with one of the same type and rating. If you are uncomfortable working with electricity, an electrician can replace it in minutes.