You need to delete the root htaccess file. If you do not see the root htaccess file then make sure whatever you are using to view files with has “show hidden files” checked/allowed. htaccess files are hidden files. After deleting your root htaccess file log back into your site and correct or delete your bad/invalid code that you added/created. Post a Security Log entry for what is being blocked so I can see what needs to be whitelisted.
If you accidentally added the custom code in the wp-admin htaccess file (wp-admin Custom Code) then delete the /wp-admin/.htaccess file, login to your site and correct or delete your bad/invalid htaccess code.