Unlike most problems with this kind of thing, the problem is that Skyrim is stuck loading up and does not actually start the game after I have installed Dawnguard for use to play, and I have never had this problem before. Please help.

Nevermind, my problem was that one save didn't work both another did.

I had this problem. If anyone is still wondering, the way to fix a save that won't load is to uninstall Dawnguard then load up the save and save your character in a different location. Then reinstall Dawnguard and it should load up fine.