We just had a discussion on this:
When designing a part or fixture I like to have a couple of reference points which will allow me to locate the origin and the X and Y planes.
If you keep track of the Line # in Carbide Motion you can have a rough idea of where to re-start at — edit the G-code file to delete most of what has already been cut (I like to back up the file to the previous major Z down move)
William