In the event you must leave the country immediately you must ultimately secure a seat on the first available flight. To do so you have several options:
Contact your travel agent from your own country and have them set everything up for you. Stay near a phone and they can call you as soon as everything is arranged. While this is being done you can also determine how to obtain transportation to the airport.
You can use the services of a Mexican travel agency who can arrange a departing one way flight for you.
You can go directly to the airport and buy a ticket on the next available flight.
NOTE: Most tour operators selling tour packages will not sell one-way tickets under any circumstances so it is better to check with airlines as opposed to tour operators.
NOTE: Sometimes the fastest way home is via a connecting route. However, due to increased security precausions, connecting flights through a country to get to another country can now take much more time.
Buying a one-way ticket is generally more expensive and you will not be refunded for your other ticket although you may be eligible for a small refund from the airline if the reason you are purchasing the ticket is for a death in the family. In order to obtain this refund you must obtain the necessary froms from the airline as well as a death certificate.