If you think that the plugin just installed is causing the problem, you can temporarily disable the plugin.
Assuming you have authorized FTP access to the site, look for the new plugin's folder in the wp-content/plugins folder of your site. The folder should have the same name as the plugin.
Rename that folder to something else (like 'my-plugin' to 'xxxmy-plugin'). Then try to log in again. Since the plugin folder is not correct, you might see a message about an invalid plugin; ignore it for now.
Assuming you can login, then the plugin might be the issue. If you can't log in (you get an invalid user/pass), then try the 'lost password' process. If resetting the password doesn't allow you to log in, then you've got problems in the user database (or someone hacked/disabled your user account).
Assuming you have CPanel access via your hosting place (assuming the site actually belongs to you and you have authorized access), you may need to go into the users table and fix your user id and authorization level via PHPAdmin. But that's beyond the scope of this answer (and there are many tutorials on how to do that).
Hoping that it is just a bad plugin. Anything more than that tends more to a 'hacked' problem. Again, assuming your site, your ownership, and nobody else has admin-level authority of your site.