Yesterday, one of my students, Dhritiman Jana was in the lab and he spent around few hours of his time in solving the issue. Following are his note on how to get to the grub prompt:
Once you get to the command prompt, say startx. From there, you could start and reboot the system
Once the computer starts rebooting press any key constantly. When I say constantly, it should be literally constantly without a pause. At the same time, just time your tapping so that you tap once in 2 seconds or so. Then you will enter grub prompt. Press enter there, then you will enter kernel mode (There will be a list of items and the one that says kernel). Put the cursor on it and press 'e', then the screen will go back to another page, where you will see a string 'rhgb' at the prompt, put a space followed by 1 and then press enter. It may so happen that you may not see the 'rhgb' string here, in that case, just use your arrow key to get the 'rhgb' line. Once this part is taken care of, this will again take you back to kernel mode. There, this time press 'b' . Then the computer is now ready to boot in single user mode and that is root. Once the computer boots you are already logged as root. So, you can do as you please as root, and one of the things you would like to do is change the password. type passwd command and change the password to a suitable one.
Hope this solution works for all!