Everyone got logged out after the recent site hack. For me, when I tried logging in once things were back up, it would tell me I logged in, but I’d actually stay logged out.

I assumed the systems were still just down, but when I noticed others were able to log in and post without issue I tried clearing site data - that let me right in.

In Firefox or Chrome you can do so quickly by clicking the lock to the left side of the address bar.