I'm on mobile (Android 9) and since sometime last night, my left-hand sidebar has been totally missing. The page just ends on the left side before the sidebar would begin. I already tried clearing the cache but I'm still having this problem.

Click the ‘Toggle Sidebar’ button right next to the site clock at the top of the page. It will bring it back :D

THANK YOU! I have never purposely used that button before so I kind of forgot it was there... Well, problem solved. I'm just incompetent.

No problem! I sometimes accidentally hit it on mobile too, but now you know how to get it back right away :D