Answers:
I'm sure you're thinking of "Jamais Vu" (never seen). This is basically the feeling that you are seeing something totally unique and unfamiliar (even though it should be). For example, if you came home to a house you've lived in for years and you didn't recognize it.
While we're at it, presque vu (almost seen) is the feeling that you're about to see (or hear or understand) something. "Tip of the tongue" is a form of presque vu.
While déjà vu can be caused my mental illness, it's fairly common without mental illness.
Jamais vu, on the other hand, is almost always linked with mental illness or other physical/mental trauma.