We had this happen to one heifer calf this year, not all of her hair fell out but a lot of it did, much as you describe yours did. We figured it out that she was not cleaned well after she was born and the hair fell out due to the gue that was on her post birth, but she shed out with new hair and looks fine now. Give the calf time, it may grow back fine,
There is a condition where they become sensitive to light, cuased from eating moldy hay (a particular mold), and they become hyper light sensitive, if kept in a dark stall for a while and fed clean feed for a time the condition will correct its self
Then there is also a condition called Proto (for short) it is a genetic condition that is fatal in its Homzygous form, carriers are rarely affected, but an affected animal will lose hair, ears,tail, etc. and be very weak, and sensitive to sun light. no cure
good luck, if it is just a skin rash, try a bath of Listerine, it will help freshen up the skin and kill and bacteria, or fungus.
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