Dr. Peter McCullough shared his observations and what he knows of the situation. Apparently Damar experienced chest trauma during the tackle, though the existence of underlying damage to the heart muscle (such as from vaccine triggered myocarditis) could be a reason why this was severe enough to induce a heart attack/cardiac arrest. The McCullough comments seem to indicate that this disrupted blood and oxygen delivery to the brain so there would be lasting neurological effects even if his heart recovers.

https://stevekirsch.substack.com/p/what-caused-damar-hamlins-injury

: He had apparently just tackled someone, and then collapsed. I
: didn’t see that part. I just turned it on and they were
: giving him CPR on the field, surrounded by a crowd of
: players. Ambulance came on the field and took him to a
: hospital. Current status unknown. Currently they have
: switched back to the studio, where it sounds like the
: commentators are afraid he may be dead. Let us pray for
: him.

: Link:
: https://www.espn.com/nfl/story/_/id/35368372/damar-hamlin-collapses-field-bills-bengals-temporarily-suspended