On my embroidery machine, and I assume on probably all embroidery machines, there is a stitch counter that displays the stitch count where you are currently in the design.
In this photo you can see the stitch count is at #11885. If I needed to stop at this point, for whatever reason, say reloading the bobbin, or to go do something else, etc., I keep a notepad and pen by my machine and I jot down that stitch count number.
Then, if while you were away, the machine gets turned off, for whatever reason, maybe the electricity blinks, somebody accidentally hits the switch or the cord, etc., then you can return to the exact spot to finish. If you fail to do this, and the machine gets turned off, the pattern will be reset to the beginning and it will be virtually impossible to find the same spot to finish that pattern. I learned this the hard way!