A schedule change by Lufthansa, e.g. a change to the aircraft type deployed, means that no equivalent seat can be made available to you on the new flight.
An unforeseen disruption, such as bad weather, means you have to be rebooked on another flight on which there is no equivalent seat available for you.
Refunds can be requested in the LH.com service area under or through the Lufthansa Service Center, but are not possible either directly at the airport or on board your flight. Please contact Lufthansa as soon as possible after your flight for possible refunds