If your controller is not responding or seems to be frozen after it was connected to the USB port, the firmware image is most likely corrupted.

To resolve that, shortly press the recessed Update button on the back panel:

After the update button was pressed, click the "Force firmware update" button bellow.

To force firmware update on mkII controller, press and hold the right-bottom [MENU] button. While keeping the button pressed, reconnect the USB cable. The bootloader application will start on your controller.

Once in the bootloader mode, follow the instructions at Firmware update starting the step 4.