This appears to be the known problem of a broken/corrupt block-chain. The underlying technical cause is not in the GUI itself but in the ZEN core/node. 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 The workaround to do reindexing is not very convenient but still allows uses to recover from the problem.
To make sure the problem does not happen repeatedly you need to use the latest wallet version for Mac OS - 0.75.9a that includes ZENCash 2.0.11 binaries.
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.