@Thats_Me_Madz@4812T
Good answer! The only thing I'll add to that is the reason the house got unfurnished is because when you moved the sim out, you opted to take all the furnishings with you. If you do that, there will be nothing left in the home.
In order to leave it furnished, you have to choose to not take them with you. But if you do that, you might leave some personal things behind.
What I usually do, is before moving out, I take any personal stuff I want them to keep, like paintings, purchased items I want them to take with them that didn't come on the original lot like exercise equipment, post cards, decorations and such, put it all in the household inventory and sell everything else. The household inventory is taken with them.
If the house is already unfurnished, there might be something you can do. If it's a vanilla lot, one that was already there when you hit "New" game, you can start another new game, edit the lot in build mode and upload it to your library. Then, exit and load up your other save where its empty and place the lot back from your library.
If it was one you created, you can place it from your library if you managed to save a copy while it was furnished, but if you didn't, then you'll just have to furnish it again. Remember to save a copy though to your library. I always save copies of all my custom lots to the library JIC.