New save will not play after dloading lot from gallery

Jpopper07​  The lot you're using may be broken, and breaking your saves.  So don't use it.  If you want to use a Gallery build, test it in a new save first, and only add it to a save you care about once you're satisfied it works properly AND you can reload the save that contains it.

Any sim you've created should be saved to your in-game library, so at least that part of your work isn't completely gone.