All Buildings have disappeared!

In my Sims game, all buildings have disappeared on the lots. It looks as though the game has reset itself as any buildings I have built have been replaced with the previous buildings. I haven't built many new houses in Willow Creek or Oasis Springs so when a house I built disappeared, I only thought it was that one, until I went into Newcrest. Most lots in Newcrest had been built on and they were all gone. When in Oasis Springs and Willow Creek, you can see the original buildings on the map, but when you go into them, there is nothing there. I have the Get To Work and Outdoor Retreat packs and the Hospital, Police Station and Science Lab has disappeared as well. My Sims are still fine as I'm aware, only the buildings have disappeared. I wasn't too concerned about this at first, so I began rebuilding all new houses again. Everything seemed okay, until I went back into the game, everything had reset once again, I had lost all of the buildings for a second time. I haven't rebuilt anything since then and I'm not going to until I know it will stop. I do have some mods, but I downloaded them quite a while ago and I have had no problems with any of them ever.

I don't know what's happening and it is really frustrating.

Any help will be greatly appreciated.

Thank You.