zero moved

[BJB]

Can anyone help, i just carved an outline then a recess in the profile both using a 6mm End Mill. Then loaded the file for the vcarving of the names to go into the recess. But for some reason the zero has moved way off, so I have to realign the zero which is a big problem as I didn't mark zero (will do in future).
First question, is there a way to recall previous zero?
Second question, what did I do to make it move.
Thanks for any idea/advise so this doesn't happen again.