If the Music Maker was version MX or later you can either:
Download and install the version you had by going to the Magix website magix.com
click the My Account icon top right of the home page
login with the email address and password you registered the software with
select My Products
Here you will find a download link and your serial number.
Install the latest free version and enter your old versions serial number - this should unlock the extra content for the older version - do note however if the version you had is earlier than the 2016 version then some Soundpools and Instruments may not be available.
If the hard drive of the old computer is OK then it may be possible to recover the Instruments etc by removing the hard drive from the old PC and connecting it to the new PC and copying them across.