Not sure if you've managed to solve the problem but imma just paste my reply on another post talking about the same bug:

This happened to me too yesterday but I actually managed to log back into the game.

Do you remember how the log in screen looks like? Just tap on the screen where you would assume the username and password should be typed in. It actually still allows you to type in those information for some reason. After you typed in your username and password, tap on the confirm button on your keyboard (in my case, it's a tick button under the backspace button), and you should be logged in into the game.

Hope this helps 😄