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.

Naming the package you installed might help someone familiar with it to help you.
What happens when instead of using just sudo with a command in a terminal you use: /usr/bin/sudo some command?
I've seen a number of posts with regard to the login loop problem at the Ubuntu forums so, if you don't resolve this here you might try their forums.

thank you very much for the reply. Indeed /usr/bin/sudo works and I was able to edit ./profile. It turned out that some path was not correct. THank you very much for your time!