I cant reset my time capsule

If you performed a Hard Reset, the Time Capsule has been reset back to its original factory default settings. You will not be able to "see" the Time Capsule using AirPort Utility until you set it up again.

In other words, AirPort Utility will only display the Time Capsule after it has been reconfigured with the settings that it needs to work with your modem.

Click on the WiFi menu at the top of the Mac's screen

Look for a listing of New AirPort Base Station

Click directly on AirPort Time Capsule

That will start up the setup "wizard" to guide you through the setup of the Time Capsule again.

If your Mac has ethernet.. plug ethernet cable into the TC LAN port to your Mac or any other PC with LAN port.. make sure the green connectivity light turns on.. on the port itself. No light no need to do any more.. the TC is dead.

On a PC you will need to download the airport utility for windows. It does still work although not particularly well in win 10.

Do a full factory reset.. no files are lost. Make sure the computer is getting an IP address, 10.0.1.x and you should then be able to access the TC if not automagically then using top menu, file, configure other. Type the IP 10.0.1.1 directly into the box.

I had the modem replaced and I cant reconfigure to the new modem

What is the make and model number of the modem?

even though its connected via ethernet as it was with previous modem.

Whenever you change the Internet Service Provider or the modem, you need to perform a Hard Reset on the Time Capsule and then set it up again.....so the Time Capsule can pick up the new settings that it needs from your new modem.

So, once we understand exactly what modem you have, we can detail the steps to set up the Time Capsule again.

I'm sorry, if you do not see AirPort Time Capsule when you click on the WiFi menu......then the AirPort Time Capsule may not longer be operating correctly, and is defective.

Please contact Apple Support for further instructions, to see if your Time Capsule might be eligible for free replacement.

If you have an Apple Store near you, you can also take the Time Capsule in to let the support people take a look.

There are 3 different kinds of resets. See this Apple Support document for details:

How to reset your AirPort base station - Apple Support