There have been hundreds, if not thousands, of people who have been declared dead by medical people and later came back to life. Surely you know many have come forward with claims of experiences during their time of being dead. Some of their stories have been quite compelling. Whether you choose to believe them is up to you.
Question: How long were they dead without access to oxygen to the brain? This is a critical measurement, because after the brain dies from lack of oxygen the person cannot be restored to consciousness, ever.
OTOH, if the heart stops and the patient is medically dead (at that time) the brain still has sufficient oxygen to survive about 15-20 minutes without incurring brain damage. If during that time a heartbeat is restored the patient will just wake up as from a dream.
This occurs quite often when a person has a heart attack and is practically dead when he arrives in ER. After administering nitroglycerine and using electrical shock in most cases the heart can be revived and blood flow restored . During this time the brain may well be partially conscious and experiencing all kinds of disoriented experiences, such as bright lights, ghostly images, sounds, but those are not experiences from beyond the grave.
After the brain dies and brain cell destruction sets in, a person ceases to exist and no amount of artificial life supports can bring that person back to consciousness. If they manage to keep the body alive by artificial means as in an iron lung, the body will be alive but the person will be in a vegetative state.
Anil Seth recounted a story where he was under anesthesia (a chemically induced partial unconsciousness), and for the duration of the operation he ceased to be a person. When he came back he had no idea of how long he had been uncoscious. It could have been a 5 minutes, 5 hours, 5 years or 50 years. All sense of time had disappeared. Was he dead and resurrected? No, the subconscious part of the brain which keeps the body alive was always functional. It was his conscious mind which was rendered unconscious but not dead that caused him to mentally cease to exist as a person. He was just an object during that time.
The point is that when the brain ceases to function and has used all its remaining oxygen, it begins to experience catastrophic brain damage from which one cannot recover.