Well, I wouldn't exactly use the term "friendzone," since you were actually with her for a period of time (it's typically used by guys who can't get into a girl's pants so they use it to make themselves look like the victim. Typically). In any case, though, I would at the very least remain in contact if you honestly do care for her. Don't continually try to keep pushing the issue, or you can run the risk of pushing her away. Remain friends, but see if you can find out what caused the sudden lack of interest. Don't be outright about it, but don't remain dead silent about it waiting for her to just tell you. Use your best judgment and try and pinpoint the root cause. I won't go Hallmark special and tell you if it's meant to be it's meant to be or anything like that, but if you had something good that suddenly ended, there's always the chance of salvaging it. Just keep in contact with her for the time being. Better a friend than nothing, after all.
This is all coming from a guy who's spent nineteen years single, so this is all coming from reflection and introspection rather than experience. Don't know how helpful that makes any of this, but I'll help you how I can, man.