There are several things this could be, it all depends on the manufacturer of the lighting unit/controller that you are using but here are a few possible causes:
Check the fuse in the power supply. This may have blown if you have had a power surge or got any water in the fitting.

• Most ballasts run 2 tubes at once therefore you may find one of the bulbs isn’t working and the other will not start because of this. Try changing the bulb.
• The bulb you are using must be compatible and the same IEC standards as the ballast.
• The ballast may have overheated and may have cut off therefore may start again after it has fully cooled down.
• Please refer to the user manual provided with your fitting.