If you just want to get it running again, try this:
mv ~/.bitcoin ~/.bitcoin-old
mkdir ~/.bitcoin
cp ~/.bitcoin-old/wallet.dat ~/.bitcoin
Then restart the client. It will download the blockchain again and then it should work.
If you want to dig deeper into why it failed we can, but that's harder.
Thank you for this. Not only has it allowed my client to open again, but it's fixed another problem that has plagued me since 0.6.2. Now I can see my file, settings, and help options in the toolbar again. For some reason that bug happened to me I think around 0.6.2 upgrade.