私事 (sī shì) is a noun meaning 'personal matters' or 'private affairs' — things that are not work- or public-related. It differs from 隐私 (yǐn sī 'privacy'), which emphasizes information you keep secret, and from 私下 (sī xià), which is a noun for 'private matters' but also used adverbially as 'in private'. 私人 (sī rén) is an attributive noun meaning 'private/personal' (e.g. 私人财产 'private property'), not a general term for affairs.