Consult Your Owner's Manual
Many problems with sprinkler timers can be solved by letting the timer do the work. Most timers are equipped with tiny computer chips that run the sprinkler program and can run self-diagnostic tests. Most timers from manufacturers like Hunter and Rain Bird have basic troubleshooting tools that allow the timer to check itself and give you an idea of where the problem lies. At the very least, a self-test will pinpoint the watering zone or valve that's causing the problem.
Check the Box
If your sprinklers aren't working but the timer is on, the problem could be a fuse. (Not all timers have fuses, but some older ones do.) Open the sprinkler timer box to check the fuses; if any are broken, replace them. The problem could also be the sprinkler system's antenna, which is often powered by a 9-volt battery. Replace the battery and see if that helps.
Check the Wires
Most sprinkler systems have above-ground siphon valves that turn the sprinklers on and off. The valves are topped by a solenoid and wires that receive messages from the timer and prompt the solenoid to turn the sprinklers on and off. Check these wires to make sure they're not broken and that they're still attached to the solenoid. Also, use the manual water turn-on switch attached to the siphon valve to make sure the water is on and the sprinklers are working. Often the problem is a broken pipe or malfunction that prevents water from reaching the sprinklers. Doing this will also let you know if the siphon valve is working. If you still can't figure out the problem after running these checks, call a professional and have the system diagnosed.