Games that have been purchased, log in to become a trial version

I have downloaded minecraft once before, and it can be played normally, but after a few months, it suddenly changed to the demo version. Later, it was discovered that the reason may be that the serial number was not verified. The downloaded software, I plan to download it again to see if the problem can be solved, but it doesn't work! Because I must have a purchase serial number and I have to verify the ability to download the official game software, but why I was able to download it successfully before suddenly changing the serial number to the demo version and then the serial number cannot be verified? (The serial number is bought at a convenience store)