It does indeed read as though you’ve ended up with your normals flipped. One way to check this, I believe, is to turn on “backface culling” (I believe that in my version of Blender this is found in the “Properties” panel of the 3D view, accessible by pressing ‘N’, specifically in the section labelled “Display”): if your model suddenly looks hollow (or even disappeares entirely), then your normals are probably inverted. As heek_ suggests, either flip them or – perhaps safer – have Blender recalculate them.