In 2000, a movie was released which featured traditional american music in its soundtrack. as a result of the movie's popularity, there was a sudden increase of interest in bluegrass. which movie was it?

The answer is O Brother Where Art Thou?

The film by The Coen Brothers has a bluegrass-heavy accompanying soundtrack that is played prominently throughout the movie. The songs from the film have been cited as influencing a resurgence in modern-day folk, roots and Americana with such bands as Mumford and Sons becoming popular. This resurgence can be termed as millennial bluegrass.