Without knowing more about your equipment and environment, it's hard to say.
If you're using a laptop, for example, it's possible that some function lock
combination has altered how your computer is interpreting the arrow keys.
It's also possible that you've activated a different keyboard language, and
that has changed the key definitions.
But, assuming you've rebooted to rule out gremlins and Ctrl+left/right still
aren't working, press Ctrl+Shift+plus sign (the one on the number pad). This
should turn the mouse pointer into a cloverleaf pointer. Now, press
Ctrl+Left. This should bring up the Customize Keyboard dialog with a command
highlighted. It should say WordLeft. Does it?
If Ctrl+Left/Right have somehow become reassigned to something else, you can
reassign them in several ways. The easiest way to restore default keyboard
settings--assuming you don't have customizations you want to preserve--is to
delete or rename Normal.dot (Normal.dotm if you're using Word 2007 or 2010).
Barring that, you can customize the keyboard and reassign Ctrl+Left and
Ctrl+Right to WordLeft and WordRight, respectively. The precise steps would
depend on which version of Word you're using. To get started, in Word 2007,
click the Office button, Word Options, Customize tab, Customize button (near
the bottom).
