If your Crave wallets gets shut down unexpectedly, you may encounter this error with the blockchain files.
If you are running a GUI wallet, you should see a popup message that asks, "Do you want to rebuild the block database now?" Say 'OK' to this, and your wallet will proceed to resync.
If you are running on a Linux-based system, you may also see that same message in the command line when you try to start the wallet. To fix this, close down the wallet and start it again with the following variation (may depend on the directory you are working in)...
./craved -rescan -reindex