Just like at home, a good first step is to contact the local police. You'll then want to call the claims number you received when you bought your Mexican auto insurance to report the incident.
As mentioned above, Mexican authorities may take you into custody before determining fault in a car accident. If you're found at fault, you may be held in custody until it's determined that you're able to pay for damages — another great reason to make sure you have adequate coverage.
Mexico's version of 911 is 066 — call in case of a roadside emergency. The State Department warns that this number isn't always answered.
If you find yourself stranded or in danger on the cuetos, or toll roads, you can call upon Mexico's roadside-assistance organization, the Green Angels (Angeles Verdes). They're a bilingual crew of radio-dispatched truck drivers that offers supplies, first aid, car care, and protection. You can alert them by calling 078.