Two things to check are the PATH variables for yourself and root and whether the coreutils package is installed. Your system may not even run without coreutils. So maybe some commands got deleted.
jschiwal@hpmedia:~> rpm -qf /bin/su
Make sure that /bin/ is in your path.
Also check the permissions of su. It should be owned by root and have the suid bit set:
-rwsr-xr-x 1 root root 37320 Jun 6 2008 /bin/su
If something happened to /bin/su, try to fix it using sudo, or logging into a virtual terminal ([CTRL]-ALT-F2 for example) to fix the problem.