An adult who suddenly collapses and is not responsive is likely experiencing cardiac arrest. The chance of survival is very low unless someone calls 911 for help and immediately starts cardiopulmonary resuscitation.
Since 2008, the American Heart Association has recommended hands-only CPR for adults, although people trained in mouth-to-mouth resuscitation can also provide CPR with breaths.
To perform hands-only CPR, put the heel of one hand in the center of the chest, and then put your other hand on top of the first. Push hard and fast in the center of the chest until help arrives. It's important to push at a rate of at least 100 beats per minute.
Keep shoulders positioned directly above the hands. Arms should stay straight.
Source: American Heart Association