Nikon D5 100 entry machine, no autofocus, it was ok before, how to adjust it?

See if you accidentally touched the automatic and manual setting buttons on the lens. M is manual and fully automatic. If you don't need to try another lens, determine whether it's the lens or the fuselage. Usually there is a big problem with the lens. I suggest you try Nikon lens in a physical store. It can be automatic, so the problem lies in the lens.