Sometimes, you need to restart Apache (after making a configuration change) or your POP3 service is not working properly. WHM has some quick links to restart services, but what if cPanel needs to be restarted?
SSH into your server using an SSH client (PuTTY is free for Windows as is Xshell, MacOS has the built-in "ssh" command in Terminal.app).
Here are some of the various services you can restart:
cPanel/WHM - service cpanel restart
Apache (Web Server) - service httpd restart
Exim (SMTP Server) - service exim restart
Courier (IMAP/POP3 Server) - service courier-imap restart
FTP Server - service pure-ftpd restart
MySQL (Database Server) - service mysql restart
BIND (DNS Server) - service named restart
The whole VPS (when not able to reach Virtuozzo) - init 6 (or shutdown -r now)
Webmail is part of the cPanel service, and uses the authentication services of the currently setup IMAP service such as Courier-IMAP or Dovecot.
service cpanel restart
service courier-imap restart
service dovecot restart
Depending on the nature of the issue, there are other 'quick fix' scripts that can be run. Contact Support for help with these.