What to do when gnome-shell freezes?

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http://askubuntu.com – Everything works correctly (music is still playing etc.), just the gui freezes. I am able to ctrl+alt+f1. What can I do after logging in the ctrl+alt+f1 terminal, in order to restore gnome-shell? If gnome-shell worked, I'd just hit alt+f2 and run r. I usually end up sudo rebooting which I don't like, because I have to reopen apps. (HowTos)