Most likely it is corrupt. What is the size on it? Microsoft says that a .PST will not corrupt until it reaches a size of 2GB, but I am an Exchange Administrator for my company and I deal with this issue on a weekly basis. I have found that for whatever reason it can corrupt at any size, but as it starts getting near 1GB it almost always corrupts. Outlok actually comes with a utility called scanPST.exe, which should be located on your harddrive at C:\Program Files\Common Files\System\Mapi\1033\NT. I haven't had a whole lot of success with that particular utility, and I actually use some third party recovery software I got my company to buy thats pretty expensive. You may just want to do a search on google or something for PST repair and can probably download a free trail version of something that will work since you'll probably only need it one time. Hope this helps.