[SixOfSpades]

Immediately go back to your oldest Save. If this was before people turned Hostile to you, thank the gods and your lucky stars, because being a Fallen Paladin sucks. If, however, you made the poor choice of Saving your game after suffering this fall from grace, I have heard that there is a way to have your "Fallen" status revoked, and it involves the magic words "Paladin Stronghold." (I've never Fallen myself, so I can't really claim that the thing actually gets you your original class/kit back.)

The most logical cause of this is, as Azaghal said, that you harmed or offended some Innocent character (by Failing a Pickpocket, opening a Locked container in their home, hitting somebody with an Area-of-Effect spell). Whenever that happens, all "Innocent" characters will turn Hostile to you, and stay that way for the rest of their lives.