Changed Username, Broke Everything
When you changed your username, did you also change the name of your home folder. Without more information, my best guess would be that you changed your username by going to System Preferences→Users &amp; Groups, right clicking on your account and choosing "Advanced Options". There are fields there where you can change your username (inexplicably labeled "Account name") and your home directory.

You should never change either of these for the account you're currently logged in as! Log in as a different user.

Changing the "Home directory" field only changes where the account looks for its home directory. It does not actually rename the folder. 

Continuing to guess, I will surmise that that's exactly what you did. You wanted to change your username from oldyou to newyou and at the same time changed the name of your home folder from /Users/oldyou to /Users/newyou, but made the change only in the dialog, not on disk. The next time you logged in, the system noticed your home folder (/Users/newyou) was not present, so it made one for you as if for a new user.

The good news is that your old home folder, /Users/oldyou, is still there with all your data (Minecraft included), exactly as you left it.

There are two possible paths you can take here, both depending on your still being able to perform admin functions.

One method is to go back to System Preferences and change the "Home directory" field back to /Users/oldyou. You'll have to restart, but when you log in all your data will be back. You'll have a now-useless folder /Users/newyou, which you can delete.

It'll mean that the name of your home folder no longer conforms to the  /Users/username convention, but that should be OK. It's just a convention, and only poorly written software will get confused by it. Sadly, we live in a world where poorly written software exists.

Which leads to the second method for resolving your problem. If you can still create accounts, make a new admin account. In that account, delete /Users/newyou and rename /Users/oldyou to /Users/newyou.

These folders are marked as deny delete, which also means you can't rename them. (Renaming is tantamount to deleting and then adding.) Finder probably won't even let you try. You can do it from Terminal, though, by first:

sudo mv /Users/newyou /Users/trashme
sudo mv /Users/oldyou /Users/newyou


Be sure you've replaced newyou and oldyou with the proper names. Verify that you can log in to your newyou account. Once you're sure that everything is working as it should, you can delete the /Users/trashme folder with:

sudo rm -rf /User/trashme


Be very sure you've entered that exactly as I've given it. Copy/paste might be a good idea. Punctuation, including spaces, is very important here.