While doing the initial block download for a new installation of mine, my laptop battery ended and the system crashed suddenly.
The blockchain files got corrupted and I couldn't start bitcoin anymore. I've written this here: https://bitcointalk.org/index.php?topic=46916.msg605718#msg605718

Copying the relevant part of the linked message:

"After I restart and tried to load the bitcoin client again, I got two error popups, with the following messages:

First popup:

EXCEPTION: NSt8ios_base7failureE
CAutoFile::read : end of file
bitcoin in AppInit()

Second popup:

EXCEPTION: NSt8ios_base7failureE
CAutoFile::read : end of file
bitcoin in CMyApp::OnUnhandledException()

And after clicking OK to the second popup, nothing happens, bitcoin does not start."

Workaround: Delete all block chain files and start over.