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This happens from time to time: the "hide" option is just grayed out and the usual shortcut doesn't work. I'm not doing anything special or stuck in a modal window—it just isn't available.
This is OS X default behavior in every app if you have hidden all other apps before with "Hide Others" or ⌥+⌘+H. In such a case it's not possible to hide the front most app, that's why both options are greyed out/inactive.
Here's a workaround:
If you hold the ⌥-key and click the desktop, your app will hide.
The same thing happened to me. I figured out that Google Chrome was in full screen mode on another (secondary) display. Consequently, OS X could not hide it.
Go to the Chrome to and select "Show All", the option will reappear.