You can't reset the code, but you can enter one to enable the head unit again. If you need to obtain a code, you'll need to go to a dealer, but you're better off just replacing the head unit with an aftermarket one.
Try to remove the head unit and look at the back. The previous owner may have left a label on it with that code. This happened to me once and I found it on the back.
best regards, Tony Sticks [url=http://www.bmwe36blog.com/]BMW E36 Blog[/url] / [url=http://www.bmwe36blog.com/forum/]BMW E36 Forum[/url]