It's just another common issue with MultiBit HD. You may find that using the "Manage Wallet" -> "Repair Wallet" option will help... others have had to do a full restore... (Exit/Switch -> "Switch to another wallet" -> "Restore" -> "I want to restore a wallet") and put your mnemonic/wallet words in...
In a pinch, if you need access to your coins, you can install Breadwallet (iOS/Android) or "Simple Bitcoin Wallet" (Android only) and use your mnemonic/wallet words to restore your wallet there and get access to your coins.
If none of those methods work, you'll have to do it the "long way"... read here (Options 2, if you're OK with Python scripts... otherwise Option 3.) :
https://bitcointalk.org/index.php?topic=1965348.msg19590212#msg19590212In either case, I would recommend you migrate to a new desktop wallet like Electrum. MultiBit HD seems to be, for all intents and purposes, abandoned by the developers.
Lot's of unresolved issues and no updates since March.