If you have root access through the shell (not recommended), or have set up a different user for shell login, and have the root password, you can log in remotely as root and change it there.
Otherwise you or someone else with physical access to the server can restart the server in single-user mode (which does not require a password) from the data center. How you do that differs between OS Distributions; you can find out how from a forum supporting your OS.
Jeff