One of my remote client was having this issue with her outlook. Outlook was working fine with the email one day ago and now it suddenly stopped working. After searching a little bit and Googling around I got the solution. First I would like to discuss the reason of this error, why this thing happen on outlook and suddenly outlook stop working.
This thing mostly happen to people who use the exchange account in outlook and then delete the exchange account and configure the regular e-mail account in outlook. Outlook save the address book in the exchange format that is in name format and link it with the account. Many people on the internet suggested to repair the .PST file and few other things but none of them worked for me. What actually worked for is given below:
Close the outlook and find the .nk2 file under the user profile in windows XP (For windows vista please consult the windows tech net)
C:\Documents and Settings\username\Application Data\Microsoft\Outlook
Please replace the username with your actual name or browse to the above path, for that you have to show your hidden files and also the system protected hidden files (Show hidden Files In Windows XP)
Delete the .nk2 file
Outlook will create a new .nk2 file while opening next time.
You are done! enjoy the new account in outlook