More than likely the belt failed (broke) and possibly cut an oil line. The belt can fail due to age or one or more of the idler pulleys became misaligned. Misalignment is usually caused by a badly worn pulley bearing that begins to heat up and track asymetrically. In my opinion if the belt and pulleys are all in good operating condition the belt should not come off even if it is splash with oil. I would replace all the pulleys and install a new belt as a starting point. Then I would search for the origin of the oil leak mentioned. Replacing the pulleys and installing a belt is not that difficult or expensive. See https://www.wikihow.com/Replace-a-Serpentine-Belt for a general procedure for replacing a surpentine belt. Also do a search on this forum.