Working with electricity and water is always a dangerous situation. If you need to install pool lights underwater, it will be safest to call a licensed electrician. An electrician is fully aware of every possible safety precaution that will be best when installing or fixing pool lighting. If you must troubleshoot the issue, be sure to turn off power at the main circuit breaker and leave it off until your work is complete.
If your pool lights underwater stopped working, first check the circuit breaker for a trip. If the breaker is tripped, turn the pool lights off and then turn on the circuit breaker. If it trips immediately, you have a short circuit. For something like this, you will need to call an electrician.