Three different tools for three different situations. Reaching for the wrong one is expensive.
Reboot. Restarts the server. Try a graceful reboot first, from inside the operating system or from the control panel. A hard reboot is the equivalent of pulling the power cord and can corrupt files, so it is a last resort rather than a first move.
Rescue mode. Boots a small recovery system with your real disk attached but not in use. This is the right tool when the server will not boot, when you have locked yourself out, or when a configuration change broke networking. Your data is intact and reachable - you mount the disk, fix the file that caused the problem, and reboot normally.
Rebuild. Wipes the server and reinstalls the operating system from scratch. This erases everything. It is the right choice for a fresh start or a badly compromised machine, and the wrong choice for almost every other problem. Confirm you have backups you have actually tested before you click it.
If something is broken and you are not certain which of the three you need, open a ticket first. Rescue mode has solved a great many problems that people were about to rebuild for.
