You posted under Intermatic raintight timer.
Add a comment with your model and brand and I have extensive files on timers.
The best diagnostic is the manual override.
1) Click manual override and see if Load (lights, fan, motor) come on.
If manual override clicks on-and-off crisp and clean, but the lights do not come on, check your electric power.
If the manual override does not click crisp and clean, then the timer may be rusty, or timer may need to be replaced. Buy replacement timer and swap out timer mechanisms without changing the box.
If the manual override works fine, and Load turns on-and-off with manual override, then the clock or trippers are next suspect.
2) Check if the dial is turning as time passes.
If the dial is not turning, then the clock motor is bad and you can buy a new clock motor.
If dial is turning, and manual override turns Load on-and-off, then the tripper pins might be worn out, and you can buy new trippers.
3) To buy replacement parts: Timer door has model number of timer. Timer clock located on back of timer mechanism has identifying numbers. You can snap out the timer mechanism using metal spring locate top inside box.
Grainger sells trippers and motors:http://www.grainger.com/Grainger/timer-accessories/timers/electrical/ecatalog/N-8hi
Aplus sells motors:http://www.aplussupply.com/intermatic/timermotors.htm
Intermatic sells motors:https://www.intermaticstore.com/ASPI700W.pgm?task=menucntrl&menuopt=TM