This seems to happen from time to time, especially just after loading into a new area.

The fix is pretty simple. Hold shift and right click on your character. Then left-click on the Broadcast Command option. Choose the appropriate action (such as Follow Me or Attack Nearest). You may hear a response from your companions and they should jump back into the action.

Yep and this is particularly necessary if you have the v1.03 patch. The AI was tweaked in the patch and it seems that the global "attack nearest" command doesn't 'stick' and has to be reapplied periodically. :)