My login form seems to have disappeared I'm developing a custom theme and even though I have placed the "user login" block in a region, I can't find it and now I'm locked out.

My site is on Drupal 8.
The login block is "a" way to get logged in. Another solution to get loged in is to navigate to an URL like /user/login.

So try that, and see what happens then. Of course, if you get logged in, hurry to move that disappeared block to some region that for sure you now is OK. To QA test it, use another browser before logging out again ...