This has happened to me after Ubuntu and OSX upgrades.
A possible culprit and the reason for mine was that something in one of your "dot" profile files is no longer compatible.
So one option is to open/examine your
.bash_profile files, if they exist, and possible start commenting out stuff to see where the issue is.
Of course if you normally use a command line editor you will have to switch to another (probably gui editor) to make that change.
I'd avoid rebooting while you figure this out as you might actually then not be able to login at all. This has happened to me. Only the fact that I had another account with minimal settings (but a regular thus privileged enough) account) that saved me (i had to su and edit my dotfiles from that!).