Between just before I started the last stage of labyrthian and getting the stave and going back to Whiterun my archery skill dropped from 67 to 15. No negative active effects that I can see and going to a shrine didn't help. Any ideas? It was a long walk back and I really don't want to do it again.

next time you level archery it returns back to normal
known bug, happened to me with 1h