This question, unfortunately, has many Christians puzzled because of their lack of knowledge of the bible.  Critics say it's all just a faith matter and has nothing to do with evidence or real proof.  I say that someone needs to go back to the basics of doing their homework and understanding how information was gathered and reported during these times in the early first century.  We know that scripture plainly says several times, from different eyewitnesses, that Jesus was physically seen in the presence of the disciples and others, not as a ghost or spirit. This information was canonized because of the fact that there were so many eyewitnesses and because stories and laws were passed down by oral tradition and perfectly memorized until written down.  The facts were written down shortly after the event happened, giving no time for myth to develop.  It was no big deal for Jewish leaders to memorize perfectly the first five books of the bible, and if they were off by one jot or tittle they would have to go back and study until they got it perfect–errors were not tolerated.

In 1Corinthians 15:3-7 says,  "For what I received I passed on to you as of first importance : that Christ died for our sins according to the Scriptures,  that he was buried, that he was raised on the third day according to the Scriptures,  and that he appeared to Peter, and then to the Twelve.  After that, he appeared to more than five hundred of the brothers at the same time, most of who are still living, though some have fallen asleep."  Not only did the disciple eyewitness the resurrected Jesus, but they documented  Jesus staying with them on the earth for a documented 40 days to continue to do miracles, telling them what must take place in the near future, because their minds were still unsure.  Even doubting Thomas, who by his very name was the most skeptical of all the disciples, would not believe until he put own his hands into the nail scarred hands, and felt the wound in the Messiah's side.  Although the LORD appeared through solid walls to his fearful disciples,  he specifically asked Thomas to touch his side and feel the holes in his hands where he had been crucified three days prior.  Then Thomas believed he was God and the risen Savior. The critics try to say that they were hallucinating, but if that were true it would not have happened on such a wide scale, where so many had seen and testified to this truth. 

The other obvious thing that most skeptics seem to miss is that practically all the disciples and others had denied Jesus at the time of his capture and crucifixion; they were for a lack of a better phrase, a hundred miles and runnin'.  All of a sudden, documented throughout the cannon of scriptures, and documented by eyewitnesses and verified in the antiquities, they changed their mind and were willing to die for a supposed lie?  C'mon all you skeptics out there, you're better than that.  Only something truly miraculous could have changed the way the disciples felt, not a ghost or spirit; scripture already squashed that nonsense by proving that Jesus ate broiled fish and drank with them to prove he was real, flesh and bones, not spirit.  Another quick point is that they never found a body in the tomb.  Only the Roman guard was there to secure and protect the body from being stolen and a claim to resurrection being made by Jesus' followers.  No Roman soldier would ever dare fall asleep on duty, for they would pay for the mistake with their life, so the odds were remote at best that all of the soldiers fell asleep and none noticed a pack of lawless citizens stealing the body of  Jesus–it is what I call the numbskull theory.  Moreover, this is just a small bit of the evidence of why Jesus physically rose from the dead.  Boom-shocka-locka, all you critics out there, you can't handle the truth!