Why is it only mc demo?

I bought Minecraft in my early childhood. Today I wanted to play it again for a long time, but can only play the demo.

I'm also no longer logged in with the data with which my mother has set up at that time, on the one hand because the email address is out of date and I do not remember the password. That's because I'll just take it now. Is there a way to get my old account back?

You could contact the support. As long as you have a proof that you have bought game, everything is fine.

If you do not have proof, it will be hard