Short answers, pulled from the story.
The Nika riots began on the 10th of January 532 when two criminals escaped execution and Emperor Justinian I declared a chariot race to commute their sentences. High tax rates implemented by John the Cappadocian sparked widespread anger among the populace while allegations of corruption targeted officials like Tribonian. The factions united against Justinian after he initially backed the Blues but shifted toward neutrality early in his reign.
Emperor Justinian I faced opposition from the Blue and Green factions who declared Hypatius nephew of former Emperor Anastasius I as new emperor. Generals Belisarius and Mundus led troops that stormed into the arena to suppress the uprising while Empress Theodora dissuaded Justinian from fleeing across the sea. Procopius records that factions carried Hypatius away from home against his wishes during the chaos.
An angry crowd gathered at the Hippodrome on the 13th of January 532 for scheduled races before violence erupted during race number twenty-two. The violence lasted an entire week before any resolution emerged with fires destroying much of Constantinople including the Hagia Sophia. Fighting resumed in 565 requiring city prefect Julian to purge factions again after the initial suppression.
Procopius records around 30,000 people died during the suppression though many perished from trampling rather than soldier actions. The massacre targeted both Blues and Greens who had united against the throne while thousands died in the chaos. Fires consumed the city's foremost church while senators supporting the riot faced exile and properties were confiscated by imperial authorities.