What you have described sounds very much like a condition called telogen defluxation. This is a condition of all animals but particularly affects horses. The cause of the hair loss and this condition is likely a stressor that occured about 4-6 weeks ago - perhaps a new horse to the herd, new feed, long trailer ride, illness etc.... The good news is that in these cases, the hair tends to grow right back as normal about 2 weeks after it falls out. The other possibilities would be equine lice, external parasites, or a immune mediated skin conditions (allergies and other immune conditions). I recommed you have the vet out and do have the blood taken for assessment, but speak with your vet about telogen defluxation. Best of luck
The stressor was likely the change in environment from out to stalled. Some horses get very stressed out by this.
The hair will all fall out of the mane and possibly other parts of the body (which you may not notice as the new hairs will be growing in right behind and at different rates. So, you may not notice the whole mane fall out. It is already growing back.