Recommended workflow is something like this:
1. Deactivate the plugin by renaming it via FTP.
2. Log in as an admin, name it back and make sure it’s activated. Don’t log out.
3. Check LearnDash LMS > Settings > Support and make sure nothing is in red (at least the PHP and MySQL values).
4. Check the Front End Login settings to make sure they’re valid, and make sure there are no sitewide redirects. Trying logging in using a different browser while still logged in as an admin in the other browser. Try to figure out why you can’t log in.
That’s unfortunately as far as we can probably take things without knowing the error message, if one is displayed. If PHP version is low, your host should be able to help.