sounds like you just made a totally new account and because, you haven't bought minecraft on that account, it says you have demo version, try logging in with old password and username. Ususally to change a password you go to minecraft.net, then log in, then go to profile, then under change your details click the 'Click here to change your password' button, and also minecraft saves locally, so your saves and stuff or only saved onto your own computer, so if your friend played it on his computer, it won't mess your stuff up.
