Before going to the airport, check the time of departure (flights can be rescheduled). If your flight is cancelled or in case you are denied boarding, ask for reimbursement of the full cost of the unused ticket or segments, or rerouting to the final destination at the earliest convenience. You may also be entitled to compensation between €250 and €600 depending on the flight distance and the delays incurred when rerouted. Furthermore, the airline should inform you about your rights and the reason for the cancellation or the denied boarding. It should also provide you with assistance in relation to the waiting time (meals, drinks, communication facilities and a hotel room for the night, if necessary). If passengers feel airlines do not respond in a satisfactory manner to their complaints, they can lodge a complaint to the national enforcement body in the EU Member State (or in Iceland, Switzerland and Norway) where the incident happened. When the incident concerned an EU airline and it took place outside of an EU Member State, the passenger can lodge a complaint in the EU Member State of destination.
A list of these bodies can be found here. For more about passenger rights – including the right to reimbursement – go here.