Complex sentence: Some wineries began producing again after Prohibition ended, but serious production did not begin until the 1960s and 1970s, when small winemakers began expanding in different areas of the United States.
Split into simpler sentences: Some wineries began producing again after Prohibition ended, but serious production did not begin until the 1960s and 1970s. This was when small winemakers began expanding in many different areas of the United States.