This morning they were working fine. Now I’m getting an error when I try to stream on Netflix (through my AppleTV). I get some kind of weird message saying that my HDMI cable might not be compatible, but that can’t be it.
downrightnow.com is not reporting any Netflix problem. The fact that you’re having problems with both Netflix and Hulu points to a problem at your end, as it’s unlikely that both would be having trouble at the same time. And it’s very unlikely that a failure at the source would cause video but not audio to fail. Your symptoms do point to a possible HDMI cable issue – I’m not sure why you’re dismissing that. (I think Youtube does not use HDCP while Netflix and Hulu do, but I’m not sure about that on AppleTV.) First thing to try is unplug the HDMI cable from both the TV and the AppleTV and reseat it. Next would be to replace the cable if you have another one – HDMI cables do spontaneously go bad occasionally. Another test would be to try to stream Netflix and/or Hulu from your computer, to verify that only the AppleTV is having problems.
Unplug the AppleTV from your HDMI cable for a minute or two before reconnecting, and if that doesn’t fix it restart your router. Roku doesn’t give identical error messages, but doing one or both of these resolves similar issues 90%+ of the time. (the other 10% of the time you fix it by re-installing the network connection with your router password)