If you have changed your username you may find that you can no longer interact with blocks on the server.
To fix this problem you will need to update your username on the server.
Log in to your website account and navigate to User Settings.
Scroll down to the section Minecraft Username, make sure the username is entered exactly the same as it is in-game.
Once that is completed, click Save Settings. Wait a few moments and a message will appear in game that your account has successfully linked.