You can use the default shell by picking "Run Command" under the "Shell" menu, and typing
/bin/shOnce that starts, "cd /bin," and "ls tcsh." If the file length is zero, you have a problem.
If you want tcsh back, the only option appears to be reinstalling OS X. For some people (like me), this was trivial, and it simply repaired itself. Others lost all their settings and had to start from scratch.

