I won't be giving you a slating, as I know that mothers and fathers can do a 'flit' and it's hard when that happens.
Saying that, I do think you should have tried earlier on ~ and I'm sure a court would say the same thing. As I have said on another thread, you are paying maintenance..which to some extent SHOULD be commended ~ you ca't exactly be accused of being a deadbeat dad :o)
Is there no way at all you can find out where they are? is there some history? eg. does your ex have a special reason as to why she wants no contact with you? how is the maintenance paid?
So many questions to answer...sorry, but all will need to be answered before you can move forward.