Microsoft Excel files disappear, reappear with an older date. Also password protected. Help!!!

Question

Hello. So we've been dealing with a nightmare in our office for years. We have a bunch of projection files all mapped between each other. First they have been disappearing and then reappearing with an older date. Sometimes it's a week, sometimes it's months ago. Also, some of the files are asking for passwords that neither my assistant or myself set. Stranger still, sometimes I can get in to the files, but when my assistant tries, it asks him for a password and vise versa. We've been through 2 IT companies and no one can figure it out. Has anyone out there seen something like this? Any advice because I'm about to throw myself off a bridge! Thanks!