Households gone after reinstalling Windows?

wayyaa  You didn't need to save your households to the in-game Library.  The saves themselves would be sufficient to preserve your progress, that is if they were present.  The Library would be helpful for preserving households or builds, for example to place them in a different save.

It's worth checking onedrive.com to see whether it contains any saves or other files, either in My Files or the Recycle Bin.  If not, then unfortunately the saves are gone for good.

As for why this happened, it's possible the OneDrive sync was interrupted.  It's also possible you ran out of cloud storage, that is if you haven't paid for extra.  I'm not recommending that (and an external drive is cheaper in the long run anyway), only saying that the 5 GB free can run out quickly.