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.