My game believes that I don't own mine craft anymore since I haven't played on this account for a while.

Basically I have owned Mine craft for around 5 years and I switched accounts because I got false banned on some servers on my account I'm talking about today. I came back to this account with all the info and I logged in perfectly fine until it gave me the demo world. It believes that I don't own Mine craft anymore and wants me to buy it again. How do I fix this issue because I want to play on my current account pretty badly. I don't want to spend money on buy Mine craft again for something that I bought a while ago.