You are not trying to run a Time Machine backup at this time, are you? If you are, you must wait until the backup is complete, then wait another day or two for the files to be indexed.
If you are not running a Time Machine backup at this time......then the next troubleshooting step would be to perform a complete network power cycle.
That means powering off every device on your network.....modem, Time Capsule, computers, phones accessories printer, etc......in any order that you want.
Wait a few minutes
Power up the modem only and let it run for 4-5 minutes by itself
Power up the next device connected to the modem.....which might be your Time Capsule....and let it run for a full minute by itself
Keep powering up devices one at a time about a minute apart until the entire network is back up
Check to see if the Time Capsule appears in the left panel of any Finder window.
If not, then you may not have the correct settings for IPv6 communications set up on your network.....since Apple depends on IPv6 to discover and locate devices on the network. So, the next step would be to check IPv6 settings on your Time Capsule and any Macs on the network.
Post back if you are not sure how to do this.
If all else fails after that, there is another way to try to mount the Time Capsule disk, but it is more complicated and intimidating for most users, so that would be a last resort.
Truth be known, Apple does not really want users to mount the Time Capsule disk on their desktops, so they make it difficult. The Time Capsule was really designed to handle Time Machine backups from your Mac(s). To do that, the backups will occur automatically without having to mount the drive on the desktop.