My Dog Suddenly Is Afraid or Can't Go Up The Stairs?

My dog is a pure bred Cavalier King Charles Spaniel. He's going to be 6 in a few months and he's a healthy dog. But all of a sudden he is scared to go up stairs. He can run down them just fine but he refuses to go up. When he's at the bottom of the stairs, he barks and whimpers when we call him to come up. And when we rarely get him to walk up them, he goes up in a crooked awkward manner. This happened a few days ago, and nothing happened to his legs. We didn't drop him or step on him, he just won't go. He doesn't walk with a limp or anything. PLEASE SOMEONE HELP ME.