If You just installed Ubuntu and forgot what password you selected during the installation process or one of your friend gave you ubuntu computer without giving username and password.This tutorial will help you to recover username and password
1) Turn on your computer, and as soon as you the Press Esc to enter grub message, press the escape key.
2) Select the option that says (recovery mode).
3) Your PC will boot into a shell. Once you get a command prompt, type "passwd username" where the username is your username. If you can't remember your username, then you can type "ls /home" which should bring it up.
4) Enter a new password when prompted, and again when prompted again Type "shutdown -r now" to reboot your system,
That's it. You should now be able to login with your new password.
Fix Grub 21 Error in Ubuntu
I've just tried installing Ubuntu but when it restert for the first time, GRUB returns an Error 21. I know this means that it can't find the grub.conf file, but I don't know how to fix this. I used Windows XP's Recovery Console to get back into XP, but I can't access the GRUB files.
1. Boot your computer up with Ubuntu CD
2. Open a terminal window or switch to a tty.
3. Go SuperUser (that is, type "sudo -s"). Enter root passwords as necessary.
4. Type "grub"
5. Type "find /boot/grub/stage1". You'll get a response like "(hd0,1)".
Use whatever your computer spits out for the following lines.
6. Type "root (hd0,1)", or whatever your hard disk + boot partition numbers are for Ubuntu.
7. Type "setup (hd0)", to install GRUB to MBR, or "setup (hd0,1)" or whatever your hard disk + partition nr is, to install GRUB to a partition.
8. Quit grub by typing "quit".
9. Reboot and remove the bootable CD.
Note:- In the above procedure hd0,1 is an example it might be different in your case.