If you are locked out of your CoreOS instance and have lost access to the root account, you need to boot into single-user mode and reset the root password.
Vultr creates CoreOS instances with a root user and password. The core user has sudo access and may log in with an SSH Key, if the key was selected when the instance was created. If you can log in as core with the SSH key, reset the root login by executing
sudo passwd as core.
If you lost your SSH key, you can log in as core by editing the GRUB loader.
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.
coreos.autologin=tty1to the end of the line.
sudo passwdand follow the prompts to change the root password.