What OS are you using? Bash is a package that is installed seperately; its not built into the OS. You can find out the details about the name of the package as follows:
on Sun: pkginfo | grep -i bash
remove it using: pkgrm <package name from prev command>
install it again: pkgadd (check the man page - quite a few options)
on HP-UX: swlist | grep -i bash
remove it using: swremove (check the man page for interactive details)
install it again: swinstall (check the man page- again can be interactive)
on Linux (I have only ever used rpms): rpm -qa | grep -i bash
remove it using: rpm -e <package name>
install it using: rpm -i <package name>
Cheers!
bash reinstall