1. Verify that you are connected to the Internet and restart your wallet. If that does not work, proceed to the next step.
2a. If you are using the GUI wallet, go to Tools --> Wallet Repair --> Delete local Blockchain Folders. This will cause the wallet to restart and sync from scratch.
2b. If you are running on a Linux-based system, first shut down the wallet using a variation of the following command (depends on which directory you are using)...
...and proceed to restart the wallet using a variation of
./craved -rescan -reindex
Wait until the blockchain is sync'ed to the current block and everything should work fine.