strange time machine behavior

This is not a complaint.

About once a week, I get a message that a time machine backup failed. I've able to fix the problem by restarting the system.

About 4 days ago, I got a time backup failure message. I restarted the system, but "Back Up Now" failed. I thought the time machine drive was gone and needed to be replaced.

Then, miraculously, time machine started working! With new backups being done.

Two sets of questions:

1. Has anyone else seen this happen? If so, what ended up happening?

2. Any advice? I've never used time machine to recover a version of a file I've trashed, only to do a full system restore after I had a system failure.

Thanks.