If you are using Gnome: Restarting gnome from a TTY can occasionally break things, so the shell is no longer 'attached' to some backends for themes, power management etc (probably). If some action is possible in the Gnome Shell, Alt+F2+
r+Enter is usually the easiest way of restarting Gnome.
If you are (supposed to be) using another Desktop Environment (DE), like Unity: You have replaced the DE with another DE...
Either of these should only be temporary, and can be fixed by logging out and logging in again (if possible), or rebooting. If you can't log out etc from the shell, save any work, switch back to the TTY and run this to restart your machine:
HOWEVER it can also be possible to log out via terminal, you may want to try this first. Restarting the login screen (GDM/LightDM) can also sometimes work - e.g. for systems not using systemd and using lightdm:
sudo service lightdm restart