Which dungeon is this again? I've been doing these quests recently but I can't quite remember which tower you mean. Sometimes there are alternate ways out of a dungeon, but not always.

Have you tried to fast travel? I don't know if that works inside a dungeon.

You don't have a single saved game from before this point? I guess maybe that's one of the advantages of a PC. You don't have to worry about keeping tons of saves, because your hard drive space is huge.

Also on the PC you can open the console by pressing the ~ key and make changes to your game if you know how. You could probably teleport your character somewhere. But I don't think the Xbox version would have a console ...

I'll try to do a little more research for you.