The hood in our fish tank is a sealed unit. You can change the bulb, but the rest of it is all sealed. (German innit, natch....)
The light failed. Replacing it with a new one -no joy- changing the fuse -no joy - so, replace the hood then.
Switch on, the fluorescent bulb flickered for a bit, and then died. Took the hood off (to stop me dropping the unit in the tank) replaced the light, and switched on. Bingo.
Put the hood on and placed it over the tank. Switched on. Nowt. It was as off as only @Onoff can be....
It's all a deliberate plot to annoy me more than I already am.