Outlook stores your email and contact information in PST files which for some reason often get corrupted. Most of the times when this happens, Outlook can repair the damage itself. When it's doing this you get the "The Outlook data file did not close properly" message when the repair is happening.

But sometimes the damage to the PST file is so bad that Outlook can not repair it itself and Outlook fails to open the file. When this happens there is a repair program that comes with Outlook that you can try. The name of the program is scanpst.exe.

To run scanpst, go to Windows search and enter scanpst.exe and run the program that search finds, this will start the Inbox Repair Tool application.

Click the Browse... button to find your PST file to repair. The file will usually be in the C:\Documents and Settings\username\Local Settings\Application Data\Microsoft\Outlook directory where username is your actual user name on your computer.

Once the PST file si found, click the Repair button, cross your finger, and wait. Once it's done repairing the file you can try opening Outlook again and to see if the repair worked.