Poor girl. I would let the groomer know and have your vet take a look at Maggie. If the circulation was cut off that long, the skin is dead. I'm not sure if there is something that needs to be done to prevent additional problems in the area.
I agree with Dollydoodle that hair accessories need to be removed and re-done daily.
Topknots with band or clips should be checked carefully to make sure they are not too tight. I admit my boys help in that department by scratching at their heads if a hair is pulled too tight.
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