The very next step is to try launching in Safe Boot Mode by holding down the SHIFT key at power-on for 30 seconds, and see if that works. I am assuming at least that'll work.
If that (presumably) works, just restart normally and see if things are cured.
However, if things are still unable to log into that user account in normal mode, but you could get into Safe Boot, go back into Safe Boot and set up a test user account and verify that at least works in normal mode:
How to test an issue in another user account on your Mac - Apple Support
Post back here on the results of these tests.