In the case of denied boarding, passengers have the right to be either rerouted on an alternative flight or reimbursed of the cost of the original ticket, apart from receiving proper assistance when at the airport, as foreseen by law (meals, refreshments, telephone calls and - if necessary - hotel accommodation and transport from/to the airport). Moreover, passengers have also the right to receive compensation, which amount is closely related to the distance covered by the flight (from € 250 up to € 600).
Should the cancelled flight being part of a package travel, claimants must address their complaint both to the air carrier and to the travel agency (or the tour operator).
Please remember that, usually, the air company reimburses firstly tangible damages, if supplied with the related receipts, such as the cost of the unused ticket and the expenses for meals and travel in consequence of the ill service (e.g. car hire, bus, taxi). Anyway, apart from these reimbursements, passengers are allowed to claim for compensation because of ruined holiday, even if, at the moment, it is very difficult to obtain this type of compensation without taking legal proceedings.