I don't know when or how this happened but a couple of days ago I noticed my Marksman skill was red and down to fifteen. I'm noy sure what it's supposed to be at I think it's around 24. I've tried using a Fortify Marksman potion and I've tried unequipping everything but it's done nothing.

Check your Active Effects tab in your character's Magic menu and look for any items listed in red. You probably have some kind of disease.

If that's the case, drink a potion of Cure Disease or go visit a temple and receive a blessing and all should be well again.

And if it's like the weird bug I have every now and then, it won't be the first time you see that

It usually fixes itself in the end though, but yeah, no idea what causes it, but it mostly happens to my one handed skill.

The real joy of TES is waiting for the next bug.

If cure disease/using a shrine does not work, then the only way to fix it is to lvl up the skill once. The same thing happened to me with Archery, and i did not even get diseased.

Seems you suffered the same bug as I. ^^
Even though it says 15 though, if you go into the perk tree proper, you will see that it isn't 15, but the proper number. If it's the same bug. If not, new bug!

it's nothing.

just level up.