No planes available for booked flights!

My aunt has a booked flight yesterday afternoon and was suppose to take off at 18h30, but when she checked in they said there are not planes available for the flight that she was booked on!!! How the hell is this possible?? How can they say there are not planes available for flights that have been booked already!!! And eventually when a plane was available it had technical difficulties and a technician had to attend to it first which took almost an hour!! This is really pathetic service!! The customers should have been notified about this before hand so that the passengers can make arrangements at their destination!!