Ayutthaya, the second great Thai capital, was sacked and largely destroyed by the Burmese in 1776 (people in the U.S. shouldn't have much trouble remembering this date). Most of its monuments were stripped of precious metals and the Buddha statues decapitated.
My guess is that this one's head has been restored.
Bang Pa-In Palace was originally built in the Ayutthaya era in Thai history, but was built into what we see here by the famed king Rama V, who had a taste not only for native Thai, but also for European, architecture.