I’m frustrated

Reinstalling Nero2015 Platinum troubles

My computer had to be repaired and the operating system rebuilt. I installed the trial version of Nero2015 platinum. When I tried to play a bluray disc using the blu ray player it told me I needed to upgrade. I checked Nero control center and it wont allow me to enter my serial key, it keeps telling me it already exists on my machine, yet it shows a ? in place of where the serial number should be.
How can I start using the product I paid for after reinstalling from a trial version?
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