Bitcoin wallet failed to read block

To summarise the conclusion of that thread, you should begin by making a full backup. Then, once you are sure your backup is secure, you can delete everything except the wallet.dat file in the ~/.bitcoin folder. This will cause BitCoin to re-download the block chain and restore the local block database. Don't panic if it initially shows a zero balance after restarting--it won't show your correct balance until it's downloaded the relevant blocks.

On a side note, this is also the safest way to perform an upgrade to a new version of the BitCoin software--backup everything, then delete everything except the wallet.dat file. When you run the new version of bitcoin, your balance will appear once it finishes downloading the blockchain.