If you registered with adobe, you will find your license number in your account on the main adobe page.
In the downloads you will find previous versions there as long as you have the license number you should be ready to go.
What if someone never registered it and the box is missing.
Then you are going to need your receipt and contact Adobe. As you will need proof you own the program. With out proof, there is very little that can be done. If you purchased it directly from Adobe, they may have a record of it, if it is not too old.