In case anyone new to weaving is wondering how you recover from a mistake like this, measure out a new warp thread, thread it through the reed and heddle, and then hang it off of the back of the loom with a weight. I wrap the end of the warp around the weight until it hangs properly, and unwind it a bit every few times I advance the warp.
Then wrap the other end around a pin and pin it to the fabric. After enough rows eventually tension will keep it in place.