Hi there this is not at all difficult, firstly
1. Close outlook or better still don't open it on this sesssion, then
2. Do a file search (not sure what version your using) in the field that you say what you are searching for enter *.pst this way it should find it whatever the file name is.
3. When you have found it you can rightclick on the file name and call it say old outlook pst
4. Open up outlook this should prompt the software to tell you that the pst file is missing and offer to recreat it this you should do and hey presto your problem should go !
Good luck! :)