Suggest that you power off the AirPort Express for now and concentrate on getting the AirPort Extreme working correctly with AirPort Utility again.
Power off the AirPort Extreme, your Mac, and everything else on the network. After you power down the modem, unplug the power cord from the back of the modem or from the wall AC jack.
Wait a few minutes
Power up the modem first, by itself and let it run a few minutes
Power up the AirPort Extreme next and let it run a full minute
Keep powering up devices one at a time the same way until the network is back up
Check for proper operation
Report on your results