Is it already quilted? If so, carefully add a narrow strip of matching fabric to patch what disappeared. Otherwise, follow the other suggestions.

Putting a border on the back is very acceptable. Breathe. It will turn out OK -- maybe better.

Yeah just add a strip to that one side..it'll still look good:)

Sorry, I read the original post again and missed the fact that you were tearing a backing fabric on a quilt that had already been hand quilted!!! Can you post a photo of the backing?

We've got a language problem here. It is impossible to tear on the bias. yYou can tear lenghtwise or across the grain, but tearing on the bias is not happening!

I would straighten the backing out then try to match the other side or end. and sew a contrasting piece on both ends.. have been where you are before. It worked out great. Let us know how it turns out.

If you have the other part of the fabric, try sewing it back together but add a little insert of other coloured material or such like. You can say it was put there to denote where the top of the quilt is, if it is on the short end. Do two inserts (one on each side) if it is on the long end.

Since the fabric is already quilted and now you're just missing what appears to be a diagonal piece along one side, I would definitely cut that side to make a definite angle and attach a piece of the fabric that you used on the front. That will make it look like a design feature. Now breathe.

I haven't had enough coffee this morning, perhaps, but did you handquilt the quilt without the backing? All the answers I see imply that you can piece the backing, which you CAN do, when preparing a backing, but how is this possible once the quilt is quilted?

Good advice from everyone, but I especially like the idea of slashing the back and inserting a second color/design to make it big enough for the backing. Show us when you finish.