Q: Airport Utility

AirPort Admin Utility was used about 10 years ago. 

Hopefully, you don't have the ancient "round" or "dome" shaped AirPort Extreme. Do you?

Then, AirPort Utility 6.x on your Mac will work just great.  AirPort Utility for the iPhone or iPad will work fine as well.

Both versions meet the System Requirements for the "tower" version of the AirPort Extreme.

Power off the entire network....all devices......any order that you want

Wait a minute

Start the modem first, and let it run a few minutes by itself

Then, start the next device connected to the modem, and let it run a full minute

Keep starting devices up one at a time about a minute apart until the network is back up

Make sure that you are connected to the wireless network that the AIrPort is providing

Try AirPort Utility again

Any luck?