Four Basic Theories
1. Swoon = Jesus just passed out and woke up later...
2. Wrong Tomb = Jesus' followers got the wrong spot...
3. Theft = Jesus' followers stole the body and made up the story...
4. Vision/Legendary Development = grief stricken disciples "saw" Jesus, but it was the overwhelming emotions, and they began to believe that Jesus was among them in a spiritual/emotional sense, and the story grew.