IPv6 on your Mac might need to be changed to a different setting
To check on your Mac......
Open System Preferences (gear icon on the dock)
Open Network
Click on WiFi on the left panel to highlight it
Click Advanced at the lower right of the window
Click the TCP/IP tab at the top of the next window
Make sure that the setting for Configure IPv6 is set to Link Local Only
If it's not, change it to that setting
Click OK
Click Apply in the next window that appears
Then, perform the same steps as above except click on Ethernet first when you open the Network window
Restart your Mac
Power off the Time Capsule, count to ten, then power it back up
Check to see if the Time Capsule now appears in AirPort Utility. If it does, try to run a backup again.
If the Time Capsule still does not appear in AirPort Utility, there is really nothing left to do at this point except reset the Time Capsule back to default settings and then set it up again to see if that might get things working again.