If Outlook doesn’t shut down properly for whatever reason–and those reasons could be legion–you might see this error the next time you try to start it:
A restart of your computer will fix this problem 95% of the time. However, that’s not always immediately practical and what do you do if it doesn’t?
First, if you can’t reboot right now, open the Task Manager and make sure that Outlook.exe isn’t running. If it is, kill it.
Next, check to see if you have Microsoft Lync or Skype running. If you do, shut it down. Don’t just minimize it. Exit the program.
If those two steps didn’t fix it, you’ll have to reboot.
If Outlook still won’t start after a reboot and you are connecting to your email account via Microsoft Exchange or IMAP, and not POP3, type “%localappdata%” in the Start search (without the quote marks) and hit Enter. This will open the folder C:\Users\<YourUsername>\AppData\Local. From there, browse to Microsoft\Outlook, and delete the .ost file (leave any .pst files alone). Next time you open Outlook, it will rebuild the ost file from data on your email server, including redownloading all of your old emails.
If you can’t tell which file is an .ost file, press “alt-t” then “o” to open the Folder Options window. From the View tab, uncheck “Hide extensions for known file types” and click OK. Now you can see the .ost file extension.