This appears to be the known problem of a broken/corrupt block-chain. In most cases it can be fixed safely. Some manual repairs will be necessary.
The ZEN private keys are located in the wallet.dat file in directory:
Mac OS: ~/Library/Application Support/Zen/
To make sure your ZEN is not lost, the most important thing is to keep a backup copy of your wallet.dat file - it must never be lost! It may be copied manually when the GUI wallet/zend are not running. The wallet.dat needs to be backed up regularly (not just once - say after every 30-40 outgoing transactions) and it MUST also be backed up after creating a new Z address.
Once you make sure you have a wallet backup in a safe place, this problem is most commonly solved by re-indexing the block-chain. For details of how to reindex, see the troubleshooting guide: https://github.com/ZencashOfficial/zencash-swing-wallet-ui/blob/master/docs/TroubleshootingGuide.md