As a rule of thumb...
If the rubber coupling between pan and the sani is Black then the odds are that it is time for a new one coz the motor bearings will be in a bad way and or the plastic lid will be brittle and it will break if you remove it.
Most likely it is a sanipro. most common reason is wet wipes or rim freshner stopping the blades rotating.
Tools required. Cheap wet vac, long nosed pliars, screwdrivers, torch, 1 pair of marigolds and a small bag to put removed item(s) in.
Isolate power, remove water from pan, remove pan, remove all traces of wet wipes from blade area. turn power on, if motor runs & stops then job done, if motor hums but runs when you flick the blades with a screwdriver then you need help with electrical parts. If motor runs but doesn`t stop then either discharge pipe is blocked or you need help with internal problem. You can add water from bath / basin to test with the pan out if pump is in full working order.
Top tip, keep wet vac empty-ish as your insurance policy if things go wrong and don`t start until customer knows there is no gaurantee that you can fix it and you want £70 minimum. All being well it will take under an hour. Loads more I could say re fault finding but there is no point without the parts with you and experiance. Hope it`s an easy £70 for you.