Well, it seems you have a bit of a problem there. From what I've inferred, mojang lost your payment records. You should look around for people who have had the same issue. I can't help because i don't know a lot about mojang and how they work. Sorry.
Now, I thought it was just a bug, but when I logged on to minecraft.net, I clicked on the play in browser tab and it said in big bold letters
BUY THE GAME
Can someone please help me?