Can you please post a fix below
Is there a MeshRenderer component on the object? Is that enabled? Is the material compatible? Can you see the floor if you select another material? If the material is Opaque, is there Alpha Clipping and if so, can you see the floor when threshold is set to 0? If the material is Transparent, is alpha of the relevant input high enough to see it (vertex alpha if used, texture alpha if used, an alpha property if there is one)? If you look at the bottom of your floor (from underneath), can you see it then? If you select another mesh in the Mesh Filter component, can you see it then?