Brain damage from lack of oxygen cannot be reversed.
Once you lose oxygen to the brain your brains cells begin to die off and once those brain cells die off the brain cells are dead and when that happens those brain cells cannot be revived.
Brain damage occurs from lack of oxygen in as little as a few minutes and so if you do not get oxygen to the brain in a few minutes or so then the brain damage can get really severe.
The brain cells can completely die in all the brain and then the person can die.
So if someone stops breathing it's very important to call 911 and start CPR and get the person some oxygen into their mouth.
You should use your mouth on the other persons mouth and start compressing the persons chest to get them oxygen all while waiting for the ambulance.
Don't wait until the ambulance and paramedics get to the person to start CPR as when someone is not breathing and may die the seconds count.