Q: What happens to me at the border if I don't have a Passport?

A: You mean after the body cavity search?

Real Answer: Nothing, as long as you're not dumb.
Show your driver's license, be polite, respectful and even offer an apology.
You may be delayed for a minute while they tap the keyboard & ask you a few questions.
They will look at the others in your car.
And then they will send you on your merry way back to the U.S.A.

They may finish off by handing you one of the lovely brochures below.
You will not be exiled to the Mexican Island of Misfit Toys.
They fully expect some to not have their passports and they have no interest in keeping
legitimate American citizens from returning home.
There is no passport jail, you will not be delayed for hours, there is no fine.

Thomas Winkowski, assistant commissioner with
U.S. Customs and Border Protection says:
"We are still going to admit you if we are satisfied with your citizenship and identity."
Read the last paragraph of this Noncompliant notice.