Ok, you don’t really "hijack" or “hack” into anything in Salesforce.com, but this is how you can take over a User account of someone that has left the company and their account is inactive. The below assumes that you have an extra license, or at least one you can borrow for a moment.
So how do you retrieve a report that is hidden away in a Personal Reports folder? What you need to do is change the email address associated with the User Account to the administrator’s email address.
Reset the password on the User’s Account. The Administrator will then get then a temporary password emailed to them from Salesforce.com. Using the temporary password, the Administrator can create a new password, and now that they have taken over the User’s account.
Now that the Administrator has access to the User’s account they can get into the Personal Reports folder and get to the report that everyone desperately needed. You can then Run the report and do a “Save” or “Save As” and save or save a copy of the report out to a public folder. Problem solved!