I think you may simply have removed that backup disk from Time Machine. You can add it again: Back up your Mac with Time Machine - Apple Support. Step 2.
Since the backup drive stopped working, when Time Machine attempts to back up to it again it may fail again. You can try "repairing" it with Disk Utility.
Otherwise, erase it and start a new set of backups on that disk. It may continue to work for a long time. Replace it with a known working drive prior to erasing it. The backups it contains may still be viable in the event you need them.
This may help explain your other question: I deleted time machine off my computer… - Apple Community