Ctl+Alt+Backspace twice will restart X, but if the keyboard is locked may not work.
The next level is to use 'skinny elephants'
See here
http://www.brunolinux.com/01-First_Things_To_Know/Skinny_Elephants.htmlThat should do a clean reboot unless you have a kernel panic and are totally locked up.
It would help if you told us more about your system, hardware, and desktop environment (KDE or Gnome etc)
You should also do all outstanding updates and reboot just in case it is a problem already fixed in an update.
Finally, a common cause of random lock ups is often memory errors. Linux is more sensitive to bad memory than Windows because Linux will actually use ALL the memory available. To perform a memory test install the package memtest86+ using the software install GUI (make sure you set the two drop down filters in the top left of the GUI to All or you will not see the package)
After installing memtest86+ reboot At the Grub boot screen you will see a new item for a memory test. You should be able to run memtest indefinitely without any errors.
While we are at it, is there an equivalent to Alt-Tab to switch between programs?
That depends on the desktop you are using.
In KDE for example with desktop effects enabled you can get a really cool 3D application switcher using Alt+Tab