I think everyone (or nearly everyone) experienced a similar problem. I noticed that I did not have HGTV, Food Network, History, TLC, Discovery, and a few others, but I did have my locals, ESPN, TNT, and USA. I reset one reciever and checked another. The other receiver had the same problem, and resetting the reciever had no effect. I called DirecTV, but while I was on hold, the channels began to reappear one-by-one. About 20 minutes after HGTV reappeared, all of the missing channels had reappeared.
I hung up before I ever had a live person respond at DirecTV. That was the longest I had ever been on hold without getting a CSR, so I expect there was a problem with the entire system. It appears to be resolved now.