Side mouse button came off

Hi! First off apologies if this isnt the right sub, thought I'd give it a shot though.

Two days ago my side mouse button which I use for push to talk felt a little loose but I could deal with it until for some reason it just popped out of the mouse. Pics (sorry for the dirtiness in advance, didn't realize how bad it was until I took the pictures :P) Somehow the button is still in perfect condition and nothing on the mouse looks like its broken. I've tried sneaking it in but cant figure out how to. I'd really appreciate any advice on what to do next or where else to look for an answer, grazie!