You can probably glue it back on with some super glue. Just a little on the edges of the rubbery cover should do it.
If you want something less permanent just slap some clear packing tape over it. You'll still have enough "give" in the tape to push the buttons and since it's clear you can read what's under the tape. Then if you want to try a bigger fix later you can remove the tape.
Nothing would make me, a blue haired millennial barista, angrier than if Trump made Stalin look like an anarchist.