Xbee Factory -Hardware reset without serial communication
From Instructable, Configuring Xbees article, Un-Bricking step:


  
  Take the module out of the interface board.
  Connect the interface board to the computer.
  Open X-CTU make sure Baud Rate is set to 9600
  Go to "Modem Configuration"
  Put a check in the "Always update firmware" box
  Select proper modem from drop down menu,
  Select proper function set and firmware version from drop down menus.
  Click on the "Write" button. After a few seconds of trying to read the modem, you will get an Info box that says Action Needed. At this point, CAREFULLY insert the module into the interface board.
  You may get the info box again a short while after, just use the reset button on the interface board.