My issue may be slightly different from what others here are running into. If I turn my TV off, sometimes when I turn it back on CEC has some kind of issue reinitializing after I turn my TV back on. I've found restarting the Pi fixes this and more recently found that if I just move the HDMI cable to another port it fixes the issue as well. It's pretty bizarre.
Try switching from hdmi to livetv and back. This works for me.