This is the question Michael R. Licona asks in the title of Chapter 36 of Dembski & Licona's Evidence for God. In the second paragraph Licona quotes the apostle Paul: "If Christ has not been raised, your faith is worthless." And therein lies a problem.
Christianity places so much stock in the resurrection, Christians arguing for the truth of Christianity seem to go a bit crazy about it, clutching at the flimsiest straws to show that Jesus rose from the dead, and therefore Christianity is true. So it is with this chapter — Licona tries to show that the disciples could not all have been suffering from a common hallucination, because, he says, hallucinations don't work like that: just as people don't share the same dreams, they don't hallucinate the same events. But this isn't necessarily the case — there's such a thing as mass hysteria, for example.
Licona attempts a statistical approach:
With these things in mind, let’s consider the possibility that Jesus’ disciples, the Church persecutor Paul, and Jesus’ skeptical half-brother James experienced hallucinations of the risen Jesus. All of the twelve disciples, Paul, and James were men, who were probably of different age groups and probably of different personalities. That the Twelve were grieving is certain. Yet proposals that the disciples were hallucinating must argue that more than 15 percent of them had the experience. In fact, more than the whopping 50 percent we find among bereaving senior adults would have experienced them. Indeed, it would have been a mind-blowing 100 percent! Moreover, it must likewise be proposed that when these hallucinations occurred, they just happened to do so simultaneously. And it just so happened that they must have experienced their hallucinations in the same mode for them to believe that they had seen the same Jesus. In other words, if a group hallucination had actually occurred, it would have been more likely that the disciples would have experienced their hallucinations in different modes and of at least slightly differing content. Perhaps one would have said, “I see Jesus over by the door,” while another said, “No. I see him floating by the ceiling,” while still another said, “No. I only hear him speaking to me,” while still another said, “I only sense that he’s in the room with us.” Instead, what we have are the reports that the disciples saw Jesus.
Finally, as if his readers have already forgotten his own Chapter 33 in this book, Licona tries once more to use circular reasoning to prove his case:
There is at least one more difficult problem for those claiming that the appearances of Jesus were only hallucinations: Jesus’ tomb was empty. If Jesus had not, in fact, been raised from the dead and the appearances were hallucinations, once must still account for how Jesus’ tomb had become empty. Aside from the fact that hallucinations are horribly inadequate at explaining the appearances as we observed above, even if that were not the case they cannot account for Jesus’ empty tomb.