If a business has been marked closed on Yelp but you don't think this is accurate, first check to make sure there isn't a "Moved to" or "Renovated" banner that directs you to the new business page.
It's also possible that the business was simply showing "Closed" because it was after hours. On the Yelp app, a red "Closed" will appear when it is outside of business hours, and a green "Open" appears during business hours. On Yelp.com, you will either see a red "Closed now" or a green "Open now" next to the opening hours.
If neither of these situations apply, you can suggest an edit to the business page.