How to reinstall gnome-terminal?

Read the next paragraph to its end before doing anything!

Switch to a TTY by pressing Ctrl + Alt + F1. It also works for F2 to F6. If something is running on TTY1 (used F1), use the next one. Log in and enter sudo apt-get purge gnome-terminal. To get back to the graphical session, press Ctrl + Alt + F7.

You might want to remove the profile files before reinstalling gnome-terminal (by typing in sudo apt-get install gnome-terminal). To do so, delete ~/.gconf/apps/gnome-terminal, if it exist on your system. I have no idea where it is on Ubuntu 15.04, so maybe google this one.

Edit: I figured out the path on Ubuntu 15.04: /usr/share/glib-2.0/schemas/org.gnome.Terminal.gschema.xml.

Most likely you ran into this problem after upgrading python2.7 to python3.5, anyway the solution is rather easy: Just open xterm and type the following:

sudo apt-get -f install gnome-terminal

or

sudo apt-get remove gnome-terminal && sudo apt-get install gnome-terminal

if it didn't work either ways though, run sudo apt-get remove python3 first and repeat the steps.

open xterm or a tty terminal, and run the following command:

sudo apt-get remove gnome-terminal && sudo apt-get install gnome-terminal