Run a Crooked Mile (1969)

A man witnesses a murder in a secluded mansion. When he reports it, there's no evidence of the murder, or that anyone was there. He finds a key on the floor and goes to his boss to get help when he's struck from behind. He wakes up in a hospital room after a polo accident to find he's had amnesia for two years, is now married, and living in Switzerland. He still has the key and the memories of the murder. He goes back to England to try to resolve the mystery.