You really have a small number of choices to solve your problem. Because you did not plan for this eventuality, you must rely on 'poke and hope' by moving the tool over the partially finished work until you feel satisfied with the alignment. You can do trial cuts on overlay stock. There are lots of tricks you can invent to get satisfactory results.
To plan ahead for disaster recovery, you need to have either:
A) two reference edges that you can keep until your final cut (easy to do).
B) a reference edge and a reference hole (easy to do). The edge gives you angular alignment and the hole gives you offset to 0,0.
C) two reference holes (harder to do)
I have recently begun to use the reference edge and the reference hole as the easiest practice to use, and it has saved my butt several times.
You also need a dial gauge, that can be easily mounted on the Z-Axis, for any reasonable accuracy in the repeat finding the reference to the X and Y axis. Your eye or a ruler is simply not good enough.
Mach 3 has an edge finder input that is incredibly nice, especially for finding the center of a hole. I made and use an aluminum corner finder that allows me to find the edge (0,0) on a rough or rounded corner reliably. It will allow me to find the corner either by the hole method or the edge method on both metallic or nonmetallic stock. It has two holes, one over the 0,0 of the corner, and the other at 0.5, 0.5 from the corner. If you don't use the Mach3 edge finder feature, then set it up and use it. You will love it.
I have created several macros (various 0,0 finder routines using either edges or holes) that I can drop into my g-code programs that allow me to preset the reference every time I run the job. Now each job has built in disaster recovery. As long as I don't move the work piece, I only need to find 0,0.
good luck,
Bill