The Elder Scrolls V: Skyrim

What reduces your one-handed skill from 87 to 15?

What reduces your one-handed skill from 87(doesn't have to be 87 this is just the current situation for me) to 15?
As I said i don't know how this happens although when it does get reduced the number becomes red and going to a shrine and using it doesn't fix it and it wont go away

Its a glitch, and i don't know if the cause is known. If it happens, all you need to do is level up whatever skill is glitched, and it will revert to normal. If it happens to a skill that is already at 100 (and therefore can't be leveled again) then i think you're outta luck.