Can't Minecraft play despite the purchased version?

I've had the purchased version of Minecraft since about 2015, but now it says on the page that I can only play the demo version. I have already logged out several times and also tried the Mojang account, but it only says that I did not buy Minecraft.

Do you know how I can fix this?

This is a problem that often happens when you first log into the demo account before buying Minecraft. The easiest is when you register again.