How do I fix my cos key on my graphing calculator?

Everything else is fine on this calculator , except when I press the cos key , it won't type it out or even work no matter how hard I press on it. And I even bought new batteries and it made no difference , yes it is kinda used, but not really assuming it's that old since it's not that scratched up or anything (I bought it online used) it's a ti-86 calculator