HELP! What do I do when my Minecraft dies on me? (more information on the problem below)?

Well, I went online and followed the instructions for downloading the 'painterly pack' and that worked fine. so then I went and found another well known pack call the... 'lb something something pack'. can't really remember. But it's supposed to make everything really detail. The Yogscast used it before if you've heard of them. Anyways, i went on and switched it over to the new pack and the screen froze then went black. and every try since then to get one, it won't let me get to the main menu. so I can't switch back. I even tried deleting the pack and the game and reinstalling the game but it still won't let me to the main menu... I've tried just about everything. I've tried deleting everything and reinstalling, tried going to the Minecraft website and accessing it through the online browser. Everything. I need help and I don't know what to do. Nothing works.