How to add back a face in the wall group

I’m new to Sketchup and learning how to 3D render. I am about to add in flooring in my render, and just noticed I may have somehow deleted my floor face from my walls group. Not sure as I have my walls group grouped and locked. Are there any suggestions on how I would add back the floor face, without deleting my 3D items?

Thank you,

Unlock the wall group (see the options in the right click context menu), enter its context for editing (double click on it) and trace one edge of the floor perimeter (which seems to be part of the wall group. This will have SketchUp recreate the deleted face.
Close the wall group and lock it again.