Press Alt+F2 or Meta+R to open the Run dialog and restart Orca by running a new instance: type orca and press Enter. This will force any existing Orca processes to exit and will then restart Orca. This sometimes has the effect of unhanging the desktop (which is usually due to an ill-behaved application).
If you cannot get to a terminal window, try pressing Ctrl+Alt+Backspace: shut down the X Window System server. This should return you to the login screen.