I've noticed my big toenails have stopped growing and one has turned yellow and has a thick ridge. What is this and what do I do? I play sports a lot

Nail fungus. Unless you had a trauma to this particular nail, this is probably onychmycosis, or nail fungus. Trauma can look similar. There are now both good topical and oral antifungals to clear this up. See your local foot specialist to see what's best for you.