I used to be having login issues lately too. It appears considered one of their information migrations was bugged in a latest improve. I used to be in a position to repair the issue by clearing my native cache and logging in once more (Settings > Apps > Pokemon GO > Storage > Clear Data, Android solely).
If you might be on iOS, you may have much less management over how your apps are managed and you will have to uninstall and reinstall the sport once more. Your Pokémon are saved server-side although, so you’ll nonetheless have every little thing after you log in once more.