I purchased Minecraft Java Edition on Windows 7, and then a few months later I upgraded to Windows 10. Now I can't play Minecraft, I've tried reading about it but can't find anything. I've tried re-downloading it, finding something online etc. It says "Play Demo" but nothing on the "play demo" bug FAQ says anything related to this.
This report appears to be a duplicate of MCL-16570, which was later resolved as invalid because this is an account issue and the bug tracker is unable to help you, as the team does not have access to the appropriate resources. Please see the Mojang Studios note attached to the parent ticket to potentially fix this issue.