These were filled with custom builds and now they are all gone.

If you look in your Gallery library, you'll need to tick the spot which says you want to see CC builds. I've had some panics about builds I've done that didn't show up until I realized that the Gallery only shows non-CC builds (and Sims) unless you tick that "CC" box.

But as Luthienrising says, beware of OneDrive. Many of us have found that OneDrive messes up our games. One of the most common things is that it replaces the old, un-updated mods for the ones you've carefully made sure to update. Very frustrating. (I turn off syncing for that reason. I uninstall it, but when Windows updates their OS, it replaces it, unfortunately.)