What happened to Jesus when he was in the tomb?
Your question lies at the center of the Christian faith.
After He died, Jesus's body was wrapped and placed in a rock tomb by Joseph of Arimathea in the evening. A large stone was placed over the entrance of the tomb. On the day after Jesus's death, the Jewish Sabbath which was a day of rest, Jesus's body presumably rested in the tomb. It did not deteriorate or begin to break down, as would normally happen to a body after that length of time.
On the third day after Jesus's death, the first day of the new week, God raised Jesus from the dead, bringing Him back to life. There is no logical human explanation for how this was accomplished because it is not a logical human event.