I came across this sentence and after a long while searching I still have no idea what it means. I'm probably just an idiot but I am clueless here.
First of all, "イッてる" is an adjective(or present continuous) form of "イク", which is basically "行く"(iku) -- to go -- but written in カタカナ(katakana). When such non-foreign words are written in カタカナ, usually it is a slang or its meaning is slightly changed.
Therefore, the sentence means "Are you gone?" but in this context, the meaning would be "Are you crazy?" or "Are you loopy?".