I had the same problem yesterday during a hike in the wild--all of a sudden, no zoom, many of the buttons wouldn't work, functionality gone corkscrew in general. After trying to return the camera to its usual level of operability by turning it off and on and off and on, removing & replacing the batter and trying different choices on the preset wheel, I decided to just give in and take pictures using what the camera would do in this extremely altered state.
At first, it would take pictures only in C! & C2, which seemed frozen at extreme zoom-in and zoom-out (I forget which choice was which zoom level), with similarly frozen or no function for Function Set and other buttons. However, as I kept using the camera, zoom and button functions re-emerged sporadically and with sometimes more sometimes less functionality---it was as if the camera had had a stroke and needed to re-learn the functions it had lost. Now, 24 hours later, it is sometimes more functional, sometimes less, but never so impaired as it was at first. A couple of times it has been fully functional. So, I will continue to follow the camera, as it were, rather than trying to impose something on it---LET it get better rather than trying to MAKE it get better; it seems to have some kind of learning capability.
Of course, I am a psychologist, so this approach might not work for everybody. Engineers, for example, might have to take the whole thing apart and rebuild it with little tiny screwdrivers, slide rules and other obscure engineer ju-ju. I'm just here to say that a kind, patient and supportive approach seems to be working---camera wants to be frozen zoom? OK, let's see what kind of pictures we can take with it in that state...as it gains confidence and is not abandoned for experimenting with its controls on its own, it may build a better, more sympathetic relationship with you and you may discover ways of taking pictures you had not thought of in your previously top-down, vertical management approach. Computers, cameras and all things circuit-board are our little brothers! We must be kind to them!
Some of this is, of course, hyperbole. But it really does seem to be getting better (i.e., back to "normal", if you believe in that sort of thing) as I only take pictures as well as I can with the camera in its altered state, without trying to fix it.