Lost Access to Minecraft

I suddenly no longer have access to Minecraft on my computer. I was literally playing with my friends no less than a few weeks ago and now I tried to log on and play and I only have access to the demo. Nothing has changed so far as I know, so I can only imagine there is some sort of glitch with my account information. I still have my receipt/order confirmation, and when I log into the website and sign in with my Mojang account I can see that I still have access, but I can't log into just my old Mojang account on my computer. I am unsure as to how to tie the two accounts to one another, and really don't know who to contact or where to go with this issue, so I thought reporting a bug was the best way to go about it. I hope there is something to be done, or at least an explanation. Thanks in advance.