Surface showed up out of nowhere

Suddenly a surface that does not exist showed up on my model. I tried to close and open the file again but it still shows up. Is there anything I can do to fix it?

This is a bad display mesh caused by a bad trim somewhere. If you extract the surface that represents the base of your walls, it will likely disappear. If you use cap to remake that bottom surface it may reappear… So you will need to find the bad spot that is causing this. Without the model it’s difficult to tell where it might be…