LOL,
RF seems to have the correct suggestion for you in this case.
Having chosen to buid with a prefab and the painted it as such you simply seem to have lost the walls on the second level.
Go and choose segments to place and look for the wall only piece of the prefab you used - you should see the prefab has been placed into the segment choice list as separate pieces.
Choose the wall segment you need to repaint - in the view move up to the second level where your walls are missing and repaint (replace them).
That should do it. Dont panic you will soon crack it.
Cant find my smilies now there are to many to choose from :