If anything goes wrong during your holiday which is not your fault (e.g. if a sightseeing tour is cancelled or the hotel is not as described in the brochure), you should contact one of the following as soon as possible: the organiser, or, if applicable the local representative. They should make immediate efforts to solve the problem.
If the problem persists, gather as much evidence as you can (photos, for instance) and file a complaint once you get home. The travel agency which sold you the package may also be your first point of contact. In some countries, there are deadlines for filing a complaint about your holiday.