This problem is caused by invalid IP Address blocking .htaccess code in your root .htaccess file.
Example – there is no space between “from” and “14” below and there should be 1 white space between “from” and “14”.
# BLOCK/FORBID Spammers etc
Order Allow,Deny
Deny from 14.146.75.
Deny from 14.146.82.
Deny from 14.147.73.
Deny from14.147.78. – there is no space between "from" and "14"
Allow from all
Screenshot of the Broken WordPress Dashboard/Menus