Not sure if this is a software or hardware issue. Several times over the years my TM drive has failed. Different drives and on different Macs. One day it just appears as a new disk needing to be initialized.
I don't have this issue with other drives except when they are truly failing, but TM drives seem prone to this while the drive will (once initialized) last for a long time afterwards.

Last week my studio iMacs external TM drive did the same thing. I keep all pertinent work files and photos and invoices on an external that goes home with me every day... and all files also reside on my Drobo in the office.... so restarting TM isn't a big deal.... but it's worrisome that all these years later TM still has this issue.

On another note.... I may have just realized what happened. Lightning struck a pole last week and the whole building got power surges a couple of times before the power went down. I'm a dolt and don't have a UPS on my Mac. Maybe it was backing up at that moment? Even so.... aren't the TM backups incremental, it should still see all data prior.

I need to buy a UPS.
Off to Staples