My Minecraft is not more, suddenly only demo version?

I've had a minecraft account for about 3 years now. I bought it on Ebay and at first everything was fine and good but now I only have the demo version. Has my account started or so does anyone know what's going on there?

Maybe the previous owner changed the password (if you have not already done so, that's very likely)
You can try logging in to the MC Launcher again and see if that works
If nothing should go then turn to the Mojang support
(VERY unimportant) Your account has been stolen

This happened to me the other day. If you re-install minecraft then it should work. Hope i helped.