Matthew 12:39-40New International Version (NIV):

(39) He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah. (40) For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth.

Jonah (Peace-Be-Upon-Him) was thrown into ocean during huge storm. He should have died, BUT he survived and probably he was unconscious.

Same way, Jesus (Peace-Be-Upon-Him) was crucified on the cross. He should have died, BUT he survived and he was unconscious. People were in rush and thought he is dead and so buried him in hurry.

Jonah (Peace-Be-Upon-Him) was in the belly of big fish for three days and three nights, should have died, BUT he survived and he was unconscious.

Same way, Jesus (Peace-Be-Upon-Him) was buried into grave and lived there for three days and three nights. He should have died, BUT he survived and he was unconscious, later got little strength to come out of it.

Later fish thrown out Jonah (Peace-Be-Upon-Him) at a beach, where he was unconscious, later survived.

Same way, pious Christians found Jesus (Peace-Be-Upon-Him) and help him recover and so he survived.
 
References:

Jesus says that the sign will be the sign of Jonah
Jonah
https://en.wikipedia.org/wiki/Jonah

Jonah? Meet the Man Who Survived Three Days in a Giant Whale’s Belly
http://howafrica.com/meet-man-survived-three-days-whales-belly

How could Jonah survive three days in the belly of a “whale”?
http://christiananswers.net/q-eden/edn-t004.html

Only Jesus is God BUT not Jonah
Some Bible Verses that say “Jesus is God”
http://www.global-right-path.com/Ummah-Reformer/MR_Jesus_Jonah_Survivals_Jesus_is_God.htm

Did God die? If Jesus was God, and Jesus died on the cross, does that mean God died?
http://www.global-right-path.com/Ummah-Reformer/MR_Jesus_Jonah_Survivals_Did_God_Die.htm

Important Note:
http://www.answering-christianity.com/isaiah_53.htm

Pronounced dead, man takes ‘miraculous’ turn
http://www.today.com/news/pronounced-dead-man-takes-miraculous-turn-2D80555113
 
'Miracle recovery' of teen declared brain dead by four doctors
http://www.telegraph.co.uk/news/health/news/9223408/Miracle-recovery-of-teen-declared-brain-dead-by-four-doctors.html

Coma man wakes after 19 years
http://www.dailymail.co.uk/health/article-187920/Coma-man-wakes-19-years.html

Coma
https://en.wikipedia.org/wiki/Coma

Lazarus syndrome (autoresuscitation after failed cardiopulmonary resuscitation)
https://en.wikipedia.org/wiki/Lazarus_syndrome

SAY: Allah is Unique; Allah is Eternal / Absolute / Independent / Self-sufficient / free from any kind of needs or association and everybody needs him); He is neither a parent nor a child of anyone; And none is like him; (Ref: Al_Quraan_112.001-004).