If the portion that's stitching out has reached the end of the 'embroiderable' area of your hoop, you will have no choice but to rehoop. I would slow down the speed to the lowest it can go and watch it like a hawk so it doesn't try and stitch on the hoop and ruin your machine. Stop it at the end of a letter. Use some sort of marking tool to mark where the hoop is before removing it. rehoop, using the markings to get it aligned. Set up a new embroidery design with the missing letters, then fiddle with the positioning to get the rest of it embroidered.
Two things for the future: Read your manual and find out how to find the 4 corners of your design. You should do this before you start stitching to see where the design will stitch in the hoop. NEVER, NEVER, NEVER leave your machine unattended when you are embroidering! find a hand project to do, or something else very close by where you have eyes and ears on the machine.