Do you own this property? If you are renting, this answer is not for you.
It is very likely that by turning the water off and removing the noted nut you'll be able to remove the cartridge, which is the inner half of the valve. Take that to a plumbing specialist supplier to see if a replacement can be found.
Hopefully then you can use the new cartridge or if you cannot find one, put back the old one and use the handle kit suggested in another answer.
It is possible, also, that while removing the cartridge you will break it, or the valve, or you'll find the valve body is hopelessly damaged, or for some other reason ... you will not be able to reassemble it. You need to be prepared for this, and willing to deal with it as follows:
- If the bathroom has its own shutoff valve, you will have to live without the bathroom for a while. Make a plan to replace the valve and probably all the plumbing serving this shower, and to refinish the wall.
- If the bathroom does not have its own shutoff valve, you will have to do all the above but also to immediately break open this wall, cut the pipes to the shower valve, and cap them so that you can turn the home's water back on while planning next steps.
It's unlikely that you won't be able to at least return the shower to its original state as pictured in your question, but you need to be aware of and willing to take the plunge if things go wrong while taking it part.
