This is how I approach troubleshooting this sort of thing.
If you cannot log in, go via cpanel
- rename plugin folders one by one and refresh the site to see if it works. this helps identify which plugin is the culprit
- check plugin requirements, might need to update php or wordpress version
- make sure the theme is up to date
- check error logs
- turn on the debugger and check that if you can
- Gemini is your friend
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_DISPLAY', false );
define( 'WP_DEBUG_LOG', true );