MacOS refuses to recognise that there is even a filesystem on the drive, and if you have to do recovery it involves just scanning the remnants filesystem looking for magic numbers without any metadata (like filenames!). I've seen it happen on three different laptops now (not all mine unfortunately, mine are at least backed up!).

The recovery tools are kind of touchy too, they only work on certain versions of MacOS and are kind of difficult to track down. The only saving grace is that Time Machine is so easy to use that people sometimes actually turn it on.