12.04 - ubuntu 14.04 stuck in login loop. Nothing seems to help

just an hour ago, I installed some package which was supposed to solve the problem with dead keys. After that I restarted my laptop and could not log in because of infinite loop with the login page.
I can use the user account but what can I do from it?

I cannot access BIOS, booting from USB does not work. I tried to switch from GUI to the shell Str+Alt+F3 and login in using my superuser credentials but sudo is not recognized in the shell. This is the error message I got:

Command ´Sudo´ is available in /usr/bin/sudo
this command cound not be located because /usr/bin is not included in the PATH environment variable.
Sudo command not found.

Im begging for help.

1. Try booting from CD/DVD or network.




2. Instead of sudo, try /usr/bin/sudo or add PATH: export PATH=$PATH:/usr/bin




3. Try booting into recovery mode: https://wiki.ubuntu.com/RecoveryMode




4. There is an option in grub to boot into single user mode (as root), usualy it requires adding single to kernel options and updating grub, but I guess it can also be done in grub command line manually.