I had to start all over again recently which was a pain.
I sold all my thieves guild armour before I was told from Mercer to upgrade it. This meant Tanilia was stuck in the dialogue telling me to come back when I'd found my original armour which meant I couldn't finish the thieves guild.
I decided to start a new character to quickly run through the thieves guild so that I could get the achievement. Unfortunately when I created my new character, I made about 2 or 3 so went to delete tho ones I wasn't going to use and accidentally deleted my level 54
I'm slowly working through the game again but the worst part is having to discover the locations all over again.
If I was nearly through the game I probably wouldn't have bothered but I'd only really done the dark brotherhood and thieves guild plus 3 main quests 1 Mage guild, 1 bards college, 3 companions and 10 side quests.
Still got a lot to do so I'll crack on with it, was nearly in tears when I'd realised what I'd done though