When you change your name, you lose access to most of the things earned on ManaCube, as you have unfortunately found out the hard way. :(
On the other hand, you are able to get things back such as your donation rank (through the
store), and I believe you are also able to get back your island on Skyblock (by asking a staff member). If you would like access to everything you had prior to your name change, simply just change your name back to what it was and you will automatically have everything back.