. . . . I need to activate my software that WAS working a few years ago. . . . .
If you are getting error message ' Error -24 serial number has already been registered to another email address' - you could, however I do not recommend this, use the old email address and password you used to previously register the product - however see item 1 below.
I would recommend that you email Magix at email@example.com and ask them to merge the old email address into the account you have created in the forum, unfortunately you will not be able to do this, in your old account as you have created the one you used here and you are allowed only one account per email address.
If you are not getting the Error -24 message: then:
If the serial number begins with P2- then the software is obsolete and cannot be activated.
What is the full name of the Magix software that was working now requires re-activation?
How long ago did you purchase the software and have it working?