They are more than likely choking or have something preventing them from getting oxygen.
If you are not certified in CPR:
Find out where you are exactly (address) or in relation to another location.
Have 1 person call your local emergency service. If there is a crowd around the person, tell everyone that only 1 person needs to dial 911 or the emergency number in your country, multiple calls just wastes emergency lines and causes the operators to spend more time answering the same calls.
Do anything the operator asks you. They may or may not tell you how to perform CPR or give aid.
Do not do anything unless the operator says so. You do not want to cause further harm.
Keep the crowd away, unless anyone is CPR certified. The more untrained people you have trying to help, can mean more people in the way.
If you are trained in CPR:
Find out where you are exactly (address) or in relation to another location.
Have 1 person call your local emergency service. If there is a crowd around the person, tell everyone that only 1 person needs to dial 911 or the emergency number in your country, multiple calls just wastes emergency lines and causes the operators to spend more time answering the same calls.
Tell the operator that you are trained and certified in CPR. This way they know that someone is able to give aid.
Tell the operator you are going to start giving aid. And tell them what you are going to do. Listen to them if they correct you on what to do.
Start giving aid/start CPR. If possible you can keep them on the phone while you give aid, and you can give them an update on the patient.