Turn off power between each action listed below.
Is dial rotating and keeping time on timer?
This will tell you if 120V is arriving and working at timer.
Cut ground wire and check circuit again.
This can narrow down ground fault problem.
Disconnect wires from timer and connect wires direct to outdoor lights.
This will tell you if timer is suspect.
List all things on same circuit breaker, and test each to find which are working.
Each electrical box is connected to next box and to next box back to circuit breaker.
Open each box and check for loose wires, signs of burning.
This will narrow down location of problem.
When working in main breaker box, the wires on main breaker have power at all times, even when main breaker is turned off. This is true unless meter is pulled. Always stand on dry boards and never lean into or touch anything else when working in main box. Do not hold screwdriver between teeth.
Tape tester leads to wood sticks to keep hands away from voltage.
Test voltage at circuit breaker. Make sure neutral and ground are attached tight on busbar. Make sure black wire is attached tight to breaker. Move black wire to another same-size circuit breaker. Disconnect ground wire and test circuit again to check if ground fault is causing problem.
Resources and illustrations.http://waterheatertimer.org/See-inside-main-breaker-box.htmlhttp://waterheatertimer.org/How-to-replace-circuit-breaker.htmlhttp://waterheatertimer.org/B220C.html