Hi lw1234,
So if the r button press feels the same as it did when it was working and still doesn't work, a wire broke off. It will have to be resoldered.
If the r button press feels different then it did when it was working, the silicone rubber pad underneath might have gotten dislodged. If that's the case you'll need to take the back plate off and reseat the silicone rubber pad into the r button.