Cranial nerve palsy. This should be evaluated by a doctor urgently. Often it can resolve, depending ion what caused it.
There are many. Double vision is often divide into painless and painful causes. Anything from diabetes to aneurysms and even brain tumors can cause the symptoms. On-eyed double vision (present even with one eye closed) is commonly caused by cataract. Regardless of the cause, double vision is something that needs to be evaluated without haste.