If it was the free license giveaway, they’re no longer valid for Unity 4 and you’re going to have to pay I think $500 (instead of the normal $800) to get them back.
I was successfully able to revert back to unity 3.5 and revive my licenses. I will have to rebuild the scenes though since Unity 4 converted them, and it isn’t set up to be able to convert back to 3.5