I suddenly can't see my left sidebar anymore and I have no idea what I clicked to make it go away. Help?

There should be a Toggle Sidebar link up near the clock (at least it's still next to the clock as far as I know. I hid mine with CSS). If you click that again, the sidebar should come back

Apparently, it's only on mobile. On my pc, it's fine. Strange.

I would maybe clear your mobile browser's cache or something, or look for that Toggle Sidebar button. I still have my sidebar on mobile :x