I was enjoying my adventures around Markarth. All was going well until these guards threw me into prison for putting my nose where it doesn't belong. I joined a gang of weary prisoners and we made our escape into some old dwemer ruins. Then they just left me down there with no key!

Does anyone know any codes or cheats that I can do to get a key, unlock the door or to teleport to Markarth? Otherwise I am stuck and I have to start the whole game over, I saved just before activating the door because I thought that there would be a lot of fighting.

So the door to Markarth is locked? That's unusual, try wiki-ing the bugs for that quest. If you have no saves to revert to I'm not sure what I can do to help.

Since your other thread was locked, and this one is identical, I wonder what will happen next.

Open the console with the tilde (~) key, click on the door and type "unlock."