honestly, it sounds to me like there is underlying, non-physical issue. it doesn't hurt to look into the physical part first though, just to rule it out. i lose my libido on both anti-depressants and any birth control pills.
emotionally, maybe start with closely examining the details of your relationship. has anything changed? are you feeling some sort of difference between you, like with arguments or more coldness or distance? have either of you become suspicious or discontented with each other?
also, it may not even be something that is within the relationship, but it could be an external factor. as in, have you moved, changed jobs, or had any difference in your friends or family lately?
because you've been together so long already, my advice would be to talk openly and ride it out. be honest that your libido has changed, and discuss your concerns about it. a multi-year relationship can weather changes like this, but without frank communication things could deteriorate.