Black Screen and/or licence

My computer was stolen. The stolen computer was running Windows 7. My son plays and loves minecraft and it was working fine. I had purchased a licence for minecraft. I have a new computer and it is running windows 10. I installed Java and minecraft. All I get is a black screen and YES I have un-installed and re-installed minecraft. I also seem to be getting indications that I must now purchase a licence for minecraft to run on windows 10? My son never plays minecraft while connected to the internet. I only want to redeem wahtever is due to me and get minecraft working on the new PC. Please can somebody assist. If this is the wrong forum the please, on my knees, tell me where to log sometjing with Mojang, because after hunting for an hour I am non the wiser.