Skyrim NPCs are attacking me for no reason

Did you steal anything and then run away? What happens if you surrender? Drop all your valuable stuff and just go to jail. If that doesn't help then it's definitely some kind of a glitch though, maybe someone here can help more.

I know of two possibilities:

1. You commit crimes against them, this can be checked by opening your journal and under "Crimes" tab check your bounty in Whiterun which Riverwood is under. If you have any bounty then it is possible that you have committed crimes against them. This can be cleared by surrendering to any Whiterun guards and serving you jail term or a few other means.
2. You are a Starving Vampire, vampirism is split into four stages of hunger. By the time you reach the last stage and is starving for blood normally friendly NPCs will attack you on sight. This can be cleared by fleeing out of sight of NPCs and feeding. If you want to avoid getting a bounty for feeding you can consider feeding on bandits instead.

According to this site, in the Bugs section, a few select NPCs will attack you for no reason. Namely Delphine, Sigrid, Alvor, Gerdur, and Faendal will attack. This is said to be due to a thief being in town when you arrive. Surrendering to a guard should fix this.

To surrender, sheath your weapon and try talking to them. Holding block should help if they won't talk normally.

Old question, but I think I have an answer on how to fix.

The same thing happened to me: I traveled back to Riverwood and everyone in the entire town wanted to attack me on sight. The guards were also making off comments about how I smelled like I was robbing graves, etc. I am neither a vampire nor a werewolf. I have no bounty in the Whiterun hold. The only thing I could think of is that a bug caused some internal flag to be erroneously set that told the game I was a criminal. If this was the case then I'd have to find a way to somehow trick the game into resetting that flag.

So I killed a chicken on purpose in front of a guard. As soon as the guard approached with the usual "hey stop right there" dialog, everyone in the town sheathed their weapons and went about their business. I paid my fine, spend the night in jail, and was able to return to Riverwood without further incident.

This problem is not limited to Riverwood. You can probably work-around this and similar "why do all the NPCs hate me now" problems by traveling into the wilderness, or at least away from the cell where the NPCs are attacking the player, and waiting or sleeping for 24 hours. My experience has been that peace will be restored and you can carry on with your stay in the formerly hostile location.

It's not always because you committed a crime or are a vampire. Sometimes Skyrim just bugs out. Just today I had NPCs at the whole Solitude Dock agro towards me for no reason. It was my first visit there. I quickly used the console to teleport to Whiterun, wait a few days (in-game of course) and went back to Solitude Docks. Things went back to normal. Also, sometimes NPCs from mods have bad faction settings. That can also cause this.