Channel 4 and many others are suddenly "Temporarily Unavailable"

Why are many of the local channels that I tape daily suddenly "Temporarily Unavailable"?  zip code 20170. The news at 7 pm that I normally tape was fine yesterday but today I see this message and I continue to see it at this hour. (8:30 pm). The other low end channels 4,5,7,8 are also suddenly "unavailable".