Update the chroot. You probably got an update to ChromeOS, and every time you do, at a minimum, you need to update the chroot. You really should update it more often. Outside the chroot, in the ChromeOS terminal, use
sudo sh ~/Downloads/crouton -u -n chrootname
obviously replacing chrootname with the actual name of your chroot, usually the distro version. Assuming you're using Ubuntu like most new users, it should be trusty or something like that, I don't really know the names of the Ubuntu versions. Read the instructions at the official crouton site
for further information. Google breaks crouton scripts regularly, and schneid regularly fixes them. The update script automatically downloads the latest version of the crouton script before updating the chroot.