I installed dawnguard and i ve reached the point were you become a vampire lord.During the attack of two vampires is whiterun i pressed z thinking that i will perform that fire shout but instead i became a vampire lord.All dudes started attacking me.I left whiterun thinking that they will become friendly as soon as i take the human form again. Now to my dismay every time i return to whiterun half of the NPCs attack me while the other half attack the attackers.Even if i use pacify they stop for a while and then they restart fighting.I can not talk to anyone
(which means that i cannot continue the MQ and capture the dragon in dragonreach) and even worse i ve noticed that many NPCs have died including a couple of merchants...

Re-load previous save.

That's okay, you can just revert to a previous save that you made before the whole mess.

Right?