"-bash: ls: command not found"
Since two commands are missing something “stomped” your path.
Before doing anything big, kicking off a backup might be prudent.
The default path on Mac OS X 10.7 (Lion) is something like this:
PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin

If you reset the path by typing the above into a shell window, and then typing export PATH it should manually let you test s local fix that window only.
This breakage is almost always something you need to fix in your dot files. For bash, edit and fix .bash_profile and/or .bashrc by undoing a recent change it just setting the $PATH you need again. The usual error is assigning a new path and not referring to the previous value of the variable on the right hand side of the =