Losing the followers is common, and dawnguard has ALOT of game breaking bugs and bugs in general that are hit and miss. Some have a perfect game, some get stuck somewhere like where you are.
Dawnguard seemed to have actually ruined the scripts of one of my games for some reason, no one was where they were supposed to be and they kind of just ended up in random inaccessible places. That I haven't heard of from everyone else so my game just got corrupted, may not have been dawnguard, but dawnguard does have script issues at times.
So as to how to fix it, you're on a console. I can't imagine any way of fixing it honestly. Others simply reset their game to an older save or made a new file if the problem continued.
They may be your only options.