hi, after failing at finding the horn of jurgen windcaller and finding a note instead, i traveled to sleeping giant inn to see the stranger, but when i got there, the whole city started attacking me!!!
now what should i do? i don't wanna play the game again.
by the way, i didn't do anything wrong and didn't kill any friend!

this is why you have to have multiple saves in the evident of something malevolent happening

Umm, I have not completed that questline yet. But may be check your statistics and bounty (press escape, go to statistics tab, and crime statistics). If you have a specific bounty it should say which hold it is associated with. If the bounty is very high, I think the NPCs start attacking you on sight.

Also, have you changed into werewolf/last stage vampire while in sight of friendly NPCs at some point?

I checked my bounty, it was a little high, but it didn't have mentioned the places i did crimes in, but i'm sure that i didn't do any crimes here.
also im a vampire, but im not sure that i have changed in NPCs sight - actually i dont know how to change. i arrived there at night, is that the reason?

Level 4 vampires will be attacked on sight by npc's. In order to lower your level feed on a sleeping person to drop you back down (you will need to feed 3 times probably). Once your level 1 vamp again you will be able to talk and sell.

http://www.youtube.com/watch?v=Lk5SkZ4rvKQ

how to know which level you are in vampirism? if its on the days you have been vampire, then i was at level 4 when i was selling things.
by the way, how can i feed on sb?

I think you should read this http://www.uesp.net/wiki/Skyrim:Vampirism and understand vampirism. If you check active effects and pay attention to in game messages you will know.

There are 4 stages to Vampirism depending on if you feed. If you dont feed they will start at level 1 and on the fourth day you will be a level 4 vampire (max). Feeding on a person will decrease this level by 1. Check your active effects setion and read the description of the disease it should tell you I believe.

also im a vampire

....

Good friends depending on their disposition may be willing to overlook your... um.. "condition". If it was in riverwood the trader counts you as a friend after finding the golden claw for example.

To feed, you need to find someone who's sleeping, sneak up to them and 2activate" them and it'll bring up a popup asking if you wish to feed or pickpocket them. That'll restore you to level 1 vampirism and people won't be able to identify you as a vampire by sight. Given sunlight burns you finding someone asleep should be relatively trivial

So you're a lawbreaking criminal scum vampire and you're wondering why the NPCs dislike you?

Those 2 things are why dude. Cure your vampirism, pay your fine (or pay with your blood) and they'll once again treat you like [censored] royalty.

Which is why I chose lycanthropy. I may smell like a wet dog, and howl at night, and have fur growing out my ears, but people in Riften think I'm a goddess.

Thanks all, but i could pass it with a bug

However if you want to stay a Level 4 Vamp you can just use the Calm spell....