All Sims and buildings disappeared after the March update...

Ison990​  The problem is that most of the relevant files are missing, or at least they're missing from the Tray folder (according to your screenshots).  You can see the files almost all have the .hhi extension, which is only one type; each saved household should also have files with extensions .sgi, .householdbinary, and .trayitem.  A saved build should have .bpi, .trayitem, and .blueprint files.

I don't know why these files disappeared, but it's possible that OneDrive deleted them, or some kind of cleaner app identified them as duplicates (since the names before the file extension are identical for a given Tray item).  Regardless, there's no way to recover the Tray items if the other files are missing—the game simply won't load them.  So if you can't find the other files anywhere on your computer, you're unfortunately out of luck here.

As for searching, please see this guide for instructions:

⚙️[PC] Sims 4 Missing User Content (saves, mods, etc.) | EA Forums - 12636780

You can search for *.trayitem to look for stray files elsewhere on your computer.  If you find any, move them to the Tray folder and see what loads.