Mayans - They're still here

"The Maya peoples never disappeared, neither at the time of the Classic period decline nor with the arrival of the Spanish conquistadores and the subsequent Spanish colonization of the Americas. Today, the Maya and their descendants form sizable populations throughout the Maya area and maintain a distinctive set of traditions and beliefs that are the result of the merger of pre-Columbian and post-Conquest ideas and cultures. Many Mayan languages continue to be spoken as primary languages today; the Rabinal Achí, a play written in the Achi language, was declared a Masterpiece of the Oral and Intangible Heritage of Humanity by UNESCO in 2005."

In short, the Mayans didn't 'up and disappear', as many ancient alien theorists make them out to have done. They spread out over time, and mingled with surrounding cultures, eventually dwindling the population.

In case you didn't know, ancient alien theorists really love to say that the Mayans were taken off of Earth by aliens, who gave them technology to build the structures that still stand today. Although that would be truly phenomenal, according to history, it's just not the case.