Boston--Hook Media Inc., an online ad agency, has filed for bankruptcy protection and is preparing to sell most of its assets to a division of Havas Advertising S.A. "We were surprised by the sudden downturn in the market," said Evan Grossman, senior VP of Hook Media. "The market downturn combined with an inability of some of our clients to pay their bills put us in this position." Hook Media had been one of the fastest-growing online ad agencies before the bottom started to fall out of the market. At its peak, the company had 100 employees. It closed its Atlanta office in February, and layoffs were also made in Boston and New York. Bankruptcy plans call for Hook to be acquired and become part of Media Contacts Ltd., part of the Media Planning Group of Havas.