I feel for you, but it can easily be fixed. Call your customer, explain what happened, either the backing was loaded incorrectly or the backing was not long enough to begin with and ask if she/he could supply additional backing. Seam the additional backing on and you're good to go again.
It is very important that the backing and top be measured correctly before loading and that we quilters do not pin the top down too far on the backing when we load the sandwich.