When a flight gets cancelled, you can't do much about it. However, you don't have to accept unreasonable alternative services.
"Your flight was cancelled, please turn to our staff for help." Passengers are confronted with such messages on a daily basis. It becomes even more disturbing if the provided alternative transportation is completely different from the one you originally booked. Travellers don't have to accept that. There are rules laid out in the European air passenger rights regulations for situations like delays, cancellations or changes of the flight schedule. The enforcement of these rights depends on your booking.
The flight was booked directly with the airline
The airline has to inform you of the changes and offer you two options: an alternative flight or the free cancellation of the ticket. If you choose the latter, the airline needs to refund you the total ticket price. This also applies if you booked a round trip (including the return flight) with the airline. If you choose an alternative flight, you don't have to accept everything.
According to the air passenger rights regulation, the rerouting has to be "under comparable transport conditions, to your final destination at the earliest opportunity". Therefore, if the airline offers you an alternative that is not soon or no alternative at all, e.g. if they want to rebook you to a flight on the next day eventhough there would be flights available from other airlines on the same day, we recommend that you insist on being booked to the better flight free of charge. If there is no flight available in a timely manner, you could also think about catching a rental car, train or bus - this question has not yet been entirely clarified. It is our opinion that you are entitled to such alternatives.
If the airline refuses to offer an acceptable subsitute, we advise you to write down that you felt forced to manage your transportation yourself and that you reserve the right to claim a refund of the costs afterwards.
It happens, unfortunately, that airlines reroute their customers to other flights even though there are still seats available at their booked flight. You should therefore check whether there are any seats available after you received the bad news. If so, you should insist on getting your original flight.
The flight was booked via an intermediary
From a legal perspective, the airline is your contractual partner for the transport and therefore in charge of customer information. It has to offer alternative flights or the reimbursement of the ticket costs. However, many airlines try to shift their responsibility regarding the communication to the intermediary. They argue that the consumer chose the intermediary for his booking and should therefore turn to him for help. Legally, this position is questionable, but the evaluation can take a long time. We always advise against booking flights through intermediarys - in reality, there is no saving of costs and in case of problems, a direct booking with an airline is easier to handle.
The flight is part of a package travel that was booked at the tour operator
The tour operator is responsible for any requests and the proper provision of your journey.
Tour operators usually reserve the right to changes in their terms & conditions. During the booking process, the consumer is informed that the flight schedule stated in the booking documents might change. Package travels are often planned a long time in advance and therefore the tour operators can't estimate which flights will be available.
Nevertheless, you don't have accept every change of plans of the operator. If it is a "significant" change, you have the right to protest against it and to request the free cancellation of the contract or the rebooking to another journey - or you could accept the change, but request a reasonable price reduction. What qualifies as "significant" depends on the travel itself (duration, flight dates, route etc.). If the flight is only rescheduled for some hours, it usually is considered to be acceptable. If your night rest is affected, it may not be.
If the flight dates have already been confirmed and the airline changes it afterwards, you have the same rights as if it were a direct flight booking (see above). If the airline changes the schedule significantly (e.g. departure at the next day) and is not willing to offer an alternative, you should insist that your tour operator (who is your contractual partner) finds a solution that fits more to the original plan.
If you are not offered an alternative, you don't have to accept the unilateral change of contract and can withdraw free of costs or request to be rebooked to another journey.