why won't my old airport express work?

Not sure what you mean by "won't work".

Won't turn on at all?

Won't allow configuration changes?

What are you trying to do with the AirPort Express?

If you could provide the Model No of the old Express.....look on the side of the Express for this.....it will be something like "A x x x x"

and

If you could provide the version of AirPort Utility that you are using.....open AirPort Utility, click on the AirPort Utility menu in the upper left corner of the screen, then click About AirPort Utility to locate the version number

We can probably help