After Jesus had been executed, the disciples were hiding away in a secret place for fear of death. The disciples were skeptical when they first heard of the empty tomb and their risen Lord. However, something changed. The disciples became fearless, and began traveling the world and speaking about Jesus' resurrection openly despite the danger to their personal well-being. Something changed that made them throw off their cowardice and take up the brave road of missionary work. They claimed that the "something" was that Jesus appeared to them posthumously. The disciples were convinced that Jesus rose from the dead. They claimed that Jesus had provided bodily proof of his resurrection at the request of Thomas.
The disciples preached the resurrection first in the same city where the empty tomb was located. This means that skeptics could go and see for themselves that the tomb was empty. This means that it would be impossible for them to maintain, for even one day, the story of the resurrection if the tomb wasn't empty. Therefore the tomb was empty. Some have claimed that a broad conspiracy theory took place, in which the disciples stole and hid the body of Jesus in order to tell the world that he had risen from the dead. There's a simple problem with this, however. Matthew 28:13 refutes the "stolen body" conspiracy theory, by telling us that the soldiers were paid to say the body was stolen. Secondly, as mentioned before, the disciples were willing to die to maintain the resurrection story. (and in fact, almost all did) A person might die for a lie as long as they thought it was the truth, but no sane person would die for a lie they made up. If the story of the resurrection was a lie, the disciples would be the most likely to know. The disciples were not insane, judging from their writings. Their approach and persuasion are not that of raving insanity, but of reasonable men. Therefore the disciples were absolutely convinced that they had seen the risen Christ.
Beyond this, the disciples had nothing to gain from making up a fictional resurrection account. Hypothetically, if Jesus were a mere man, proven by his death and non-resurrection, then the disciples were signing their own death warrants by continuing to follow a executed criminal. The establishment had already proven that they would kill to maintain their power. They even plotted to kill Lazarus previously, as John tells us. So there is nothing for the disciples to gain by making up this story, except suffering and probably death. Therefore, it's illogical that the story is made up.
Some have claimed that the disciples experienced a group hallucination. The problem is that we have no recorded case of identical hallucinations occurring simultaneously among multiple individuals. Instead, hallucinations are private, personal, and unique to each who experiences them. To speculate about group hallucinations is not supported by any modern research. Not to mention that a group hallucination would be as much a miracle like the resurrection. Those that claim this happened, are admitting that the miraculous is possible.
Some have speculated that Jesus simply swooned. This idea presents the notion that Jesus was not completely dead but instead lost consciousness for a few days. However, this doesn't explain how he escaped the tomb. It doesn't explain how a swooning, near corpse, overpowered the Roman soldiers who were guarding the tomb. Rome was strictly against letting a crucified criminal live. They had a process whereby they ensured that the one being crucified was undoubtedly, irrefutably, indubitably dead before being removed from the cross. The apostle John, who was an eyewitness, verifies that he saw blood and water pour out of Jesus side when the soldier pierced him. Again, as before, this idea is only weak speculation and isn't based on any contextual information, only the idea that the resurrection is hard to believe. The details are too compelling to ignore.