「あなたが 何の事を 言っているのか わからない」=(anata ga nan-no koto wo itteiru no ka wakaranai) "anata ga" is often left out, for example when you talk face to face. When talking with formal or kindly, we use "wakaranai no desu ga..."/"yoku wakari masen..."/etc. instead of "wakaranai". When you are embarrassed to other person, like Aiko's answer, you may use "sumimasen" before a formal sentence. ( There is else "何の事を言っているのか、見当がつきません"=(nan no koto wo itteiru no ka, kentou ga tsuki masen. )

"I have no idea what you're talking about." ー ’nani o itteirunoka zenzen wakarimasen.' You need to be careful when you say this to your Japanese friends. Because Japanese avoid to use direct expressions. I would say, 'sumimasen, chotto yoku wakaranain desuga...' (sorry, but i don't understand what you said well...)