That happened to me today. When I took it out of the washer, my dress looked like a shirt (!). I tried drip-drying and stretching the material. It took a while, but it soon took went back into shape. I wouldn't put it into water again. Rayon has a low retention, especially in when wet. HWM rayon is different and stronger. I would do more research before trying anything else.
Neatly fold your dress and submerse it in a sink of cool water. Leave it long enough to saturate. Drain the water off and gently press some of the wetness out. Without wringing it, hang it up over the bathtub to drip dry. Putting rayon in the dryer shrinks it and makes the fabric too soft. Letting it airdry lets the garment retain its shape. I have done this after machine drying a garment and it works.
I'm pretty sure that once you've shrunk this type of material, it's shrunk for life, same with wool. When it says to handwash, it means it. I have had some luck with machine washing on the delicate cycle some items that said "dry clean only", but I don't dry them in the dryer, I hang to drip dry.
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