You can accidentally turn a solid into a surface model by right-clicking on a face and selecting delete. This deletes a face, leaving behind a surface body. To fix it, first look in your feature tree for a DeleteFace feature. If you have done this by accident, you can just select that feature in the tree and delete it. If, however, you needed to delete that face (maybe an import part with a bad face for example) you can fix it by:
1) Right-click on one of the open edges, and pick Select open loop
2) Click Insert, Surface, Fill
3) You may need to play with the edge settings & options.
For smooth transitions to surrounding faces, select tangent for the edge option.
For sharp transitions (like a side of a cube) select contact.
4) Select "Merge result" & "Try to form solid" checkboxes
5) (Cross fingers for luck &) Click OK