I am an American citizen born in the United States. I am in Mexico at present. Can I use my birth certificate and drivers license to cross the border by foot if I don't have a passport?
By law, U.S. citizens are supposed to enter and leave the U.S. with a valid U.S. passport, with some exceptions. The exceptions include if you have a U.S. passport card, an "enhanced driver's license" (very few states offer these) or trusted traveler program cards (e.g. NEXUS), etc., or if you are a child under 16 you can use just your birth certificate.
As a U.S. citizen, you should be able to go to a U.S. consulate at any time and get a U.S. passport (and there are U.S. consulates in most major Mexican cities on the U.S. border) at anytime. Even if by some chance the U.S. has something really against you and don't want you to travel internationally (e.g. Edward Snowden), they will still issue you something for the purpose of returning to the U.S. (even if it's so they can prosecute you).
As a U.S. citizen, even if you do not have the documents that they want, if you were to arrive at the U.S. border and can satisfactorily prove your U.S. citizenship (your U.S. birth certificate and a photo ID should be a good start), you cannot be denied entry. They may detain you for a while to check it out, and give you a stern talking, but once they are convinced you are a citizen, they have to let you in.