I understand you are using a USB hard disk drive connected to the Time Capsule, is that correct? First, confirm that disk appears when you launch AirPort Utility: Select the Time Capsule > Edit > Disks. Make sure its icon appears adjacent to Partitions, that Enable file sharing is selected, Secure Shared Disks > With device password, and Remember this password in my Keychain is selected.
Then, "reconnect" to that disk as follows:
To reconnect to the backup device, open Time Machine Preferences, and select "Add or remove backup disk". Select the backup disk, then Remove Disk and confirm that selection with Stop Using This Disk. Then, select "Add or remove backup disk" again. Re-select the same one under Available Disks, then Use Disk. You will be prompted for the password you provided for that disk in AirPort Utility.
"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. That does not mean it needs to create a completely new backup, as if it's creating a brand new one. Your existing backups will still be there.
If you don't want to wait that long, select Back Up Now.
If Time Machine reports the backup disk "cannot be found" again, the solution requires deleting the entry for its password using Keychain Access. Write back for instructions.
A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.
Your existing backups will remain unaffected, but since it has been more than a few days since the last backup, it will take a long time to finish that incremental backup. TM will perform a "deep scan" which always takes a long time. A few hours, perhaps. If you upgraded macOS at some time subsequent to the previous backup, three or four days might be necessary during which "Preparing Backup" will appear with no other apparent signs of progress. Let it finish.
Needless to say, if you are using any non-Apple "anti-virus" junk, don't. That just goes without saying.
