So sorry this happened. You've learned a valuable lesson. Never wash your top before it is quilted. Quilting will help keep the seam allowances from rubbing and fraying out. I would suggest doing some random applique over the quilt so that it looks like you intended it as a design element.
SandyQuilter

Thanks everyone for your responses. I have a further question. My plan was to use a fleece backing and minimal quilting, but now I'm afraid that won't hold up to further washing since it's already so messed up. If I decide to try and repair this, should I go ahead and use batting and quilt it heavily?

I have had this happen on a quilt I made and on several that were donated to our local chapter of Project Linus. I still have no idea why it happened. On my quilt, only some of the fabrics frayed that way, yet they were from the same bundle. It was horribly discouraging.

Novicequiltergrandma, you can use the fleece alone, it will be fine. The problem isn't with or without batting. Good luck to you!

You could use the iron on interfacing and then place a strip of contrasting color over all the seams to make it look like that was the way the pattern was supposed to be. A lot of work, but it would save the top.

For fabric that frays so easily, I would definitely plan on a lot of quilting.

willferg, I am wondering how heavily quilted your Project Linus quilts were. If you use a batting such as Warm and Natural and outline quilt lines 10" apart, this does not always provide enough support for fabric seams. Usually an all-over pattern with moderate spacing (say, 2-3" apart) is sufficient to prevent fraying.

I like any of the ideas above that will salvage the quilt top... Cute appliques will cover the areas you repaired... would suggest using just the Fleece and you can quilt it in lines lengthwise about 3 to 4 inches apart. then bind edges.this would be a Quilt loved no matter how much more Love ( work) you put into it... Don't worry , just finish it and give it with a Hug...

I once made the mistake of washing an unquilted top and the same thing happened to mine. This is how we learn NEVER to wash an unquilted top! I went over all the seams that were frayed close to the stitching or had come apart, and took them in another 1/8" or more. It was a mess and made the quilt top more puckery. When I quilted it I used a serpentine stitch over every seam line, and you would never know it ever had a problem! The seams are strengthened further by that serpentine stitch holding down every seam allowance.

Good luck!

If you choose to fix it - good luck.

If you choose to not finish it - it really is okay.

Many of us have had 'learning experiences' that have ended up in the 'there is no way I can salvage this' pile. I sure have.

Sort of like the very charred dinner - it went into the garbage instead of trying to eat it.

Never wash a quilt before it is quilted. I KNOW! I used light weight fusible strips, butted the seams together and pressed, then put a small applique over it. I fused because I did not want it to fray more. I never did that again. Lesson learned.

why not try a double sided tape,they have some i 1/4 incl thin strip tapes all you need to do is apply it bu removing paper from tape, press with iron and let it set close and done .after if you what you can use a applique if you what too. i have use this tape for pant hems and have use it on quilts that come undone with washing ,if it still just the top quilt you can sew from other side of quilt to secure tape hope this helps you