Your custom installation of
bash is no longer present after upgrading to Yosemite.
If you moved Homebrew aside to make the installation of Yosemite go faster, you can move it back via the Finder and things will work.
Alternatively, you can change the shell used by Terminal.app back to the system-provided
bash by selecting
Terminal > Preferences from the menu bar (this should still be available even though the Terminal window shows it failed to start) in the Terminal.app and under the
General tab set the
Shells open with: setting to
Command (complete path): and enter:
in to the command area like so:
This will allow you to open a Terminal window. Once this window is open I strongly suggest you set your default shell back to the system-provided
bash by running:
chsh -s /bin/bash
Once you do this, you can change Terminal.app's preferences back to
Default login shell.