none of the commands work in linux terminal

All of a sudden I can't execute any command in my debian terminal:

pi@rpi ~ $ ls-bash: ls: command not found

or

pi@rpi ~ $ sudo omxplayer -r -o hdmi mymovie.mkv-bash: /usr/bin/sudo: No such file or directory

or

pi@rpi ~ $ nano text.txt-bash: nano: command not found

Nothing works. A minute before everything was fine, I was watching a movie, didn't run anything else. I can't even reboot it properly as reboot fails as well. Hard reset is coming... Any idea?

UPDATE: I tried to disconnect the ssh session but couldn't connect again. Had to turn it off and back again. It's OK now but this is not the first time this happened. I would really appreciate some tips.