Most likely is the problem.
I have seen some replacement photo cells available, if they would work for you not sure.
I usually end up just replacing the whole unit. They are cheap enough.
If I am charging the customer by the hour, for the time I spent working on old light, they could have just got a new one for same cost.
But you can try to replace the photo cell, maybe it is just dirty and needs cleaned.
Try moving the switch back and forth to testing, change the sensitivity of the switch and maybe fiddling with it is all it needs to make it work again.
If it was easy, we would not call it work!