A source has said today that once the plane was lifted with a crane, they tried to extend gears and everything worked. Nose gear deployed immediately and main gear did after manually opening the damaged doors. They are evaluating the chances of repairing the plane.
Also, Barcelona ATC confirmed that no emergency call was made from the cockpit. That seems to be confirmed by the fact that the following landing plane (a Clickair A320) had to go around when on short finals, as an eye witness has pointed.
Unless on-board recorders point to another direction, the most probable cause is said to be that the crew just forgot to extend gear. Really strange when we are talking about a captain with 5000+ flying hours. We'll wait for the official report.