First generate a new self signed certificate from bash. Once that's active, restart httpd and the GUI should come back up.
Word of caution, if you are using DNS/GTM you need to make sure the new certificate is in the trust store on all other units. Otherwise your sync would break.
Use the command below to generate a new self signed certificate for management:
openssl req -x509 -nodes -newkey rsa:2048 -keyout /config/httpd/conf/ssl.key/server.key -out /config/httpd/conf/ssl.crt/server.crt -days 3650
Once the above command has been executed successfully, restart httpd using command below:
bigstart restart httpd
Once the above steps are followed, the GUI should be accessible via browser.