I have never got this problem, so I have no idea why it happens and how to fix it.
https://en.bitcoin.it/wiki/Running_Bitcoin-salvagewallet Attempt to recover private keys from a corrupt wallet.dat
If you are using Linux, run "./bitcoin-qt -salvagewallet".
If you are using Windows, make a shortcut for your bitcoin.exe and append "-salvagewallet" to the target, and then open the program using that shortcut.
If you have any bitcoin in your wallet, do
NOT delete your wallet.dat, or you will lose your bitcoin. If you have no bitcoin yet, feel free to delete the wallet.dat file.