If you are locked out of your Debian instance and have lost access to the root account, you need to boot into single-user mode and reset the root password. These steps apply to Debian 9 and 10.
Access the instance console in the Vultr customer portal.
Click the Send CtrlAltDel button on the top right of the web console.
You can also click the Server Restart icon.
As the system boots, press ESC to bring up the GRUB boot prompt. The prompt happens early in the boot process. If you miss the prompt, you need to restart the server from the control panel and try again.
init=/bin/bashto the end of the line.
mount -o remount,rw /to mount the system volume.
passwdand follow the prompts to change the root password.