What is the output of the command '/usrbin/echo $PATH'?
It sounds to me like you may have altered your $PATH variable value in such a way that it now points to nothing. Unless you altered one of your login files (.login, .bashrc, .profile, etc.), this should fix itself if you log out and then log back in.
To edit your path to include a bin directory in your /home directory for example, you would enter the command 'PATH=$PATH:/home/yourlogin/bin'
If this doesn't work for you, let us know.