I logged in to my account that I always used, and I only have the demo now

I logged into the same Minecraft account I always used, however, it keeps telling me that I only have the demo now, and suggests I buy it for the 3rd time, because this has happened before.

What I expected to happen was...:
I expected to be able to open Minecraft and play the way I did literally the day before.

What actually happened was...:
I opened Minecraft and the button that always said "Play" changed to "Play Demo" and I was told to buy the game again.