If a casualty is not breathing it's important to call 999, start CPR (chest compressions) and get a defib (AED) on them as soon as possible, the chance of survival increase dramatically. You do not need to be qualified to use a defb and they promt you every step. All of our courses include defib training. Here is how to use one.
https://www.youtube.com/watch?v=UFvL7wTFzl0