Buy a new one. And dig that one up and make sure it is nice roots. If not plant the new on in less dense media.

Ashley, unfortunately you do not have the tree you had originally. What you have now is the rootstock it was grafted to which is worthless unless you want to graft another tree to it.

And pip313 is right, that soil is way too dense and in fact may be what killed it because it retained too much water and deprived the roots of oxygen.

That tree is very cold hardy and can do well out side in zone 7 or warner. keep it for its beauty but in ground. I am going to guess that your tree died from graft failure.

6b Steve

Thanks for the comments everyone. I should've changed the soil, although I didn't realise it was too dense until now! It was the soil is had when I bought it, but that was 2 years ago :(