After extending Network can't find Time Capsule

Start with the basics first....which is always a good idea to do anyway.....after you make a change to the modem or any routers on the network. That would be a complete network power cycle.

Power off the entire network.....all devices.....modem, Time Capsule, Express, computers, printers, iPhones, printers, etc......in any order that you want.

Wait a few minutes.....4-5 would be good

Power up the modem and let it run for a full minute by itself

Power up the Time Capsule and let it run a full minute

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

Depending on how many devices that you have, it might take 15-30 minutes to complete the entire power cycle

Check the network again and report on your results

Power off the AirPort Express and leave it off.

Work on getting the network back to the way that it was before you tried to add the AirPort Express. In other words, the network layout in AirPort Utility will display only a Time Capsule when you have things back to the way that they were before.

Worst case, you may need to reset the Time Capsule back to default settings and then set it up again. If this is the case, you can use the same device name, wireless name and password(s) that you used before, or set things up with new names and passwords.

Once you have the network set up again with the Time Capsule in charge.....then.....it is time to try to add the AirPort Express back to the network. My recommendation would be to reset the Express back to default settings and then set it up again. I am not clear on whether the Express will connect back to the Time Capsule using a wired Ethernet cable connection, or whether the Express will connect using only wireless. The setup is different depending on how the Express will be connected to the Time Capsule.