蒙冤 means 'to suffer an injustice' or 'to be wronged'. It is a separable verb (离合词): the verb 蒙 ('to suffer/endure') and the noun 冤 ('injustice') can split when a duration or measure word is inserted, e.g. 蒙过冤 ('has suffered an injustice before'). The word is formal and often used in legal, historical, or literary contexts — someone is falsely accused of a crime or treated unfairly. You cannot put the person who did the wronging directly after 蒙冤; use a prepositional phrase like 被 or 受到 instead.