Damar Hamlin spent Monday night in an intensive care unit and remained there Tuesday in critical condition, the Buffalo Bills said in a statement.

The 24-year-old NFL player collapsed during a game against the Cincinnati Bengals Monday night, and Bills officials said he suffered a cardiac arrest.

The remainder of the game between the Buffalo Bills and Cincinnati Bengals has been postponed.

What happened in the Monday Night Football game?

With 5:58 to go in the game’s first quarter, Bengals wide receiver Tee Higgins, who was running the ball, hit Hamlin, a 24-year-old safety, in the chest with his helmet during a play. Hamlin dragged Higgins down to the ground. Hamlin stood up right after — but a few seconds later, collapsed.

Medical personnel attended to Hamlin for 19 minutes on the field, The Associated Press reported, and Hamlin received both CPR and required an automated external defibrillator. CBS Sports reported he was taken off the field in a stretcher, then put in an ambulance, where he was given oxygen, to be taken to the hospital.

The Bills said early Tuesday Hamlin had suffered cardiac arrest and his heartbeat was restored on the field, and he was transferred to the University of Cincinnati Medical Center for further testing and treatment. He was sedated and listed in critical condition, the team noted.

Fans rallied outside the hospital overnight and donated millions of dollars to his effort to buy Christmas toys for children from his hometown of McKees Rocks, Pennsylvania, outside Pittsburgh.

What is “commotio cordis”?

While it’s not yet clear exactly what Hamlin experienced, CBS News medical contributor Dr. David Agus said Hamlin may have suffered what’s known as commotio cordis, which is heart arrhythmia that occurs when someone takes a direct hit to their chest.

Describing the condition as “a confusion of the heart,” where the heart muscle pumps erratically, Agus said the energy of the blow interferes with the electrical signals in the heart, creating a new beat known as ventricular fibrillation — a fast, erratic heartbeat that does not pump blood into the brain. When that happens, Agus said the heart needs to be shocked back into regular rhythm.

“That’s why there are defibrillators on the sidelines of games, is to be able to use them to shock the best heart back into a regular rhythm,” he said.

Agus said every minute blood flow is stopped to the brain increases the potential of severe consequences — including death.

“Every minute you wait is basically 10% increase likelihood of death — so the quicker you do it, the better,” he said.

Agus said the next 12 to 24 hours will show how Hamlin’s heart is recovering — but the greater question is whether he suffered any injury to his brain from his heart stopping. The faster they were able to get his heart started, Agus said, the more likely it is he will recover quickly.

Hamlin is reportedly on a ventilator, so Agus said over the next 12 to 24 hours, medical personnel will slow down that machine to see if his body takes over and he begins to breathe on his own, allowing them to remove the ventilator.

Agus said approximately 30 cases of commotio cordis occur in the U.S. every year, including among Little League players who get hit in the chest with a ball.