Did the command prompt change to a "#" (without the quotes)? If not, you are not logged as SU. If yes, command not found implies the system can not locate the binary executable such as ps.
pwd will tell you what directory you are in, and does not (usually) require root privliges. The path root has is different than normal users.
Would you post the distribution you are using? I would also suggest you use "locate" or there is a "find file" GUI program to find out where the binarys are you wish to locate. Try switching to the directory where they are and see if that helps.