Try backing up, then removing the existing settings.dat.php and then uploading settings.dat.php.default from the zip.
It will ask you to install and re-enter your product id and mysql details but once you do that you should have all your data back (accounts and sections) as it should be stored safely in MySQL.
Check your disk space on your hosting account to make sure you're not maxed out. The file may have gotten corrupted if the server didn't let you create any more files because it was full.
Let me know what you find out.
Dave Edis - Senior Developer
interactivetools.com