AirPort disc needs repair.

No need to check the network really, if you are still seeing the disk error message. The internal hard drive on the AirPort Time Capsule has failed.

If the AirPort is 5+ years old, it probably is time to retire the device, since it would not really make sense to try to replace the hard drive on the Time Capsule without also replacing the power supply as well.

Did you get a disk error?

If you click the icon of the Time Capsule in airport utility you will see the error above.

Although we get a few cases of false readings and the disk is ok.. this happens in the Gen5 more than earlier ones, it does not happen a lot. Bob already suggested the power down for long period to cool off and see if the reading is false.. if you still get it on start up, from cold then the disk needs replacing. You cannot fix it because the SMART check in the disk has triggered. You can recover your files in some cases.. but the disk is showing serious deterioration and it is time to replace it. Fixing a bad drive is not viable.

If Time Machine is saying it cannot find or use the disk that is not so much a disk problem as network issue. There is likely nothing wrong with the disk.

Unfortunately, there is no way to repair the disk without physically removing it from the Time Capsule.

Try powering off the Time Capsule and leaving it off for at least 4-5 hours, longer if possible.

After the power down period, restart your Mac and then power up the Time Capsule again.

Hold down the Shift key on your Mac while you click on the Time Machine "clock" icon at the top of the Mac's screen

Select Back Up With Consistency Scan

Keep an eye on things and report on your results.

The only real replacement for a Time Capsule is another Time Capsule, since no other manufacturer offers a product that is a Wi-Fi router with a built in hard drive.

Although Apple discontinued the Time Capsule in April 2018, products may still be available on online sites like Ebay or Amazon.

The closest that you will come to a Time Capsule will be a router like those offered by Synology or Asus with a USB port, to which you can cannot a USB hard drive. So, you'll have a two piece solution rather than a one piece solution.

If you don't need another router, then a NAS like those offered by Synology would be worth a serious look.

Because the average useful life of a Time Capsule is about 5 years. At that point, the hard drive and power supply are both suspect. It really makes little sense to replace the hard drive at that point when the power supply is on schedule to fail in the near future as well, but it's your call.

TM that's exactly 5 yrs old

That would be AC model. You can find replacement power supplies fairly easily. But it is not at all easy to replace the hard disk. It is not built with easy access like the previous Gen1-Gen4 which all had solid computer type cables and plugs. The AC version has soft mounted the hard disk with rubber plugs that fit the seagate barracuda drive exactly. You need to be creative with other brands or even different series of seagate. And all the plugs and sockets are more like iPhone type with ribbon cables.

I have changed disk and power supply in them. And just about everything else, fan, antennas. But I don't find it easy and IMHO they are not worth wasting your time on. The older Gen4 is a better candidate if you just want something to do TM backups.

For network backups a Synology NAS is probably the best target. Apple did include backup to another Mac since High Sierra and this can work well if you have a desktop Mac.. plus laptop. For just a desktop USB plugged in a better way to go.

Some routers also include using Time Machine to USB hard disk. Synology like RT2600AC or the high end Asus for example.

I also have a TM that's exactly 5 yrs old wit a failing disk. Looking at YouTube it seems relatively easy to replace it. Why would it be necessary to also replace the power supply? Thanks!

Thanks for the advice! That was the end of the Time Capsule...