Expertise level: Medium
There are two types of reboot available through WHM: graceful and forceful.
A graceful reboot attempts to safely shut down or close as many services (mail systems, scheduled tasks, firewall, etc.) before rebooting the server. In many cases, it involves saving the data and state of the various services, in order to ensure that the services can be re-started with a minimum of problems.
A forceful reboot simply reboots the server, without shutting down services running on the server. Since none of the running data is saved, this may cause issues later on, especially in the file system. Forceful reboots are reserved for emergency cases.
Even a forceful reboot fails, servers can be restarted with an APC reboot through the Customer Hub.