Assuming you mean the version released in 2008 then you may just be lucky to retrieve your serial number from Magix as follows:
Browse to the magix.com website
Click the My Account icon - third icon top right of the home page
Login with the email address and password you registered the software with.
Click My Products
You may then see if Music Maker 14 is registered and the serial number.
If the registration and serial number are not there, then the only option is to purchase a later version of Music Maker (MMM) either the Plus or Premium.
If you decide to go for the latest version do note the following:
The Free version has limited Soundpools, Instruments and Features, you would need as a minimum the Plus version, preferably the Premium version.
Music Maker 2020 requires at least Windows 7 or later.
You will not be able to open any of the old projects - there have been many changes since MMM 14.
Soundpools and Instruments that were available MMM 14 may not be available in MMM 2020.