The exact procedure varies slightly between Linux versions, but this one will work with most.
- Go to the "Server control" page in the control panel and reboot your server.
- Go to the "VNC console" page and wait for it to tell you it was disconnected. Reload the page, and you should see the initial booting screen.
- Hit the down arrow key over and over again until you see a "grub" boot menu.
- Hit the "e" key to edit the boot line.
- Cursor to the end of the line and type " single", followed by the enter key.
- Hit the "b" key to boot the system with the modified line.
- The system should boot into a special mode that has you logged in as root. Use the "passwd" command to define a new system password, then use the "reboot" command to reboot.