I assume when you say "the fabric has come loose" that you mean a seam has opened? If not, please explain.
How did you baste it? Pins? Hand thread baste? Spray baste?
Where in the quilt is the open seam? Near the center? Near the edge?
How long is the open part of the seam??
If it's pinned or hand basted, I would remove as much of the basting as possible to gently pull the block up so you can hand stitch it. If it's near the edge (within 8 or 10") and you've not quilted the edge yet - you might be able to remove the basting to the edge to get your hand under it to get the seam sewn. Work slowly, you can do it!!
If it's spray or glue basted .... more difficult.
Does the overall design of the quilt allow for you to applique something over the open seam ... even if you put another of the same applique in the other end of the quilt to make it symmetrical?? If not ... you can stitch it closed with quilting - making sure that you catch both edges of fabric (one should be folded under the other - stitch close to the seam line) and use smaller stitches. It should hold.