See also: Sacred Oath.
A paladin tries to hold to the highest standards of conduct, but even the most virtuous paladin is fallible. Sometimes the right path proves too demanding, sometimes a situation calls for the lesser of two evils, and sometimes the heat of emotion causes a paladin to transgress his or her oath.
A paladin who has broken a vow typically seeks absolution from a cleric who shares his or her faith or from another paladin of the same order. The paladin might spend an all-night vigil in prayer as a sign of penitence, or undertake a fast or similar act of self-denial. After a rite of confession and forgiveness, the paladin starts fresh.
If a paladin willfully violates his or her oath and shows no sign of repentance, the consequences can be more serious. At the DM's discretion, an impenitent paladin might be forced to abandon this class and adopt another, or perhaps to take the Oathbreaker paladin option that appears in the Dungeon Master's Guide.