Compensation is always due when you are bumped against your will.
Check your flightDenied boarding is the strongest claim in the whole regulation. If the airline sold more seats than the aircraft has and refused you boarding against your will, compensation is due immediately. There is no extraordinary circumstances defence, because overbooking is a deliberate commercial decision, not an event outside the airline's control.
The airline must first ask for volunteers to give up their seats in exchange for agreed benefits. If you volunteer and accept their offer, you give up the fixed compensation and take whatever you negotiated instead. If nobody volunteers and the airline selects you, the full amount is owed.
You are also entitled to choose between a full refund and re-routing to your destination, on top of the cash compensation, plus meals and accommodation while you wait.
The one situation where boarding can be refused without compensation is where there were reasonable grounds, such as missing travel documents, health or safety concerns, or arriving after check-in closed.
Compensation is a flat amount set by distance, not by ticket price: €250 up to 1,500 km, €400 for longer flights inside the EU and for 1,500–3,500 km, and €600 beyond 3,500 km. The UK equivalents are £220, £350 and £520.
Distance is measured in a straight line from your original departure airport to your final destination, which on a connecting itinerary often lands you in a higher band than you would expect.
Duty of care is separate from compensation and does not depend on the cause. Once you are delayed two hours or more the airline owes you meals and drinks, two phone calls, and a hotel plus transfers if you are stuck overnight. This applies in bad weather, during ATC strikes, and in every other situation where no compensation is payable.
Airlines frequently provide none of this during major disruption. Buy your own, keep every receipt, and claim the cost back afterwards.
Work out what you are owed