Seems strange there's no option to go back and undo 'appearances removed'.
This is true, however, you can use Display states to show different appearances, then switch between them.
2015 SOLIDWORKS Help - Controlling Display and Appearance in Assemblies
If it's an older part and all you did is delete the appearances then you could open File Explorer and restore an earlier version of the part..
This is true, however, you can use Display states to show different appearances, then switch between them.
2015 SOLIDWORKS Help - Controlling Display and Appearance in Assemblies