Hallucinations do happen sometimes to highly strung people, highly imaginative or sick people and also to drug takers. Jesus Christ appeared to over five hundred people over a period of 6 weeks. Were these people all highly strung people or drug takers. No, they were common Jewish people, fisherman, housewives, tax collectors, cynics. Why would the same hallucination appear to each of the 500 people He appeared to on the mountain of Galilee.
Hallucinations generally have no objective reality.
Here we have a group of disillusioned heartbroken disciples. They were hiding behind locked door. Once Christ arose their attitude changed dramatically. Their fear of the Authorities disappeared. As we track their future lives in the Bible we learn that many of the disciples spent time in prisons, endured beatings and died for their faith.