This happens when the local copy of the block chain is damaged.
First find where your wallet.dat is stored and make a copy of it in a safe place (backup).
Then delete everything in that directory and the subdirectories, except the wallet.dat and the Sync.conf
Now restart your Qt wallet, it should start normally and tell that it is out of sync, it will then download the block chain from the start
to rebuild its database that was corrupted. Once it has re-sync'ed you are back in business.
Success!
But now it says '0 active connections to network' and my balance is at 0. What to do?