Yes, this has happened before. Usually it is just that your wallet is still syncing. The GUI sometimes gets lagged behind. If you go to your Mintcoin folder, in your computer and check the blockchain, even though the wallet itself says "not responding" if you watch it, the file size of the blockchain should be getting bigger every few seconds or so. THis means your wallet is still syncing and once it is caught up, the GUI (graphic user interface) of the wallet should become responsive again. Just let your wallet stay open and keep running, at it should eventually correct itself without any issues.