There are differing approaches needed for these two very different collapses.
Contact Collapse: For the most part, a coach will see a head or abdominal injury happen (with aerial challenges being the most dangerous plays). Be warned in this contact case to look for progressive or worsening symptoms. A coach should watch out for neck pain (do not move the player), numbness, vomiting, blood in the urine or an altered/confused state. These symptoms can present and worsen, or they can cascade — meaning additional symptoms keep appearing. These are all warning signs of a possible concussion and should be addressed by a doctor or health-care provider.
Non-Contact Collapse: A non-contact collapse is considered to be the worst scenario. It usually means that the player has suffered a neurologic or cardiac event. 911 should be called immediately. If it is a cardiac event though, there is no time to wait for the EMS. Heart compressions (CPR) should be started immediately. Hands-only CPR at 100 compressions per minute could mean the difference between life and death. Even better would be the proximity to and use of an automated external defibrillator (AED). Appropriate action in the first five minutes after a non-contact collapse translates to a 75% chance of survival.
NOTE: There is a chance that it will be a coach or a referee who has a cardiac event. CPR training should be provided for coaches and referees, of course, but also for players.