This article explains the steps to reset lost root password in Linux
Step1: Reboot the server.
Step2: Highlight your image and press E to edit.
Step3: Find the line starting with "linux" and append rw init=/bin/bash at the end of that line.
Step4: Press Ctrl + X to boot.
Step5: Type in passwd username.
Step6: Set your password as in OPTION 1 above and restart your PC. Your new password should work now.