Viacom Inc. said an impasse in contract talks with DirecTV is costing almost 20 million subscribers access to 26 channels, including MTV, Nickelodeon and Comedy Central. DirecTV dropped the channels late Tuesday when two sides couldn’t reach a new fee agreement, New York-based Viacom said in a blog post. There was no word on whether the companies would continue negotiating. “We are hopeful that DirecTV will work with us toward a resolution, and stop denying its subscribers access to the networks they watch most,” Viacom said.