This problem could not be fixed and Alonso was without DRS until the finish.
DRS is usually reliable. The only notable failure of it before was the Mercedes system on Michael Schumacher's car being stuck open and forcing his retirement in Canada last year.
DRS was introduced to the sport in 2011 as an overtaking aid and concession for the F-duct which had been banned. Under certain conditions the driver is allowed to lift the rear flap open, to create a 50mm gap between the flap and the wing's main plane. This boosts top speed, allowing for easier overtaking on the nominated straights.
Teams use the car's hydraulic system to open the flap. This requires pipework leading up from the gearbox to an actuator on the wing.
Over the years the position of the actuator and how it opens the flap have changed.
Initially most teams had the actuator in the rear wing support pylon and it simply pushed upwards to open the flap. This was soon discarded and the actuator mounted inside a pod mounted above the wing. This kept the negative aerodynamic effect of the hydraulic actuator well clear of the wing.
Inside this pod the actuator will either pull the wing open directly or with a small rocker. This will be arranged to ensure the wing opens at no more than the 50mm gap stipulated in the rules.
When the hydraulic pressure is applied to the actuator the piston inside moves forwards in a few milliseconds, pulling open the wing in the process.
When the hydraulic pressure is released, the air pressure on the flap allows the wing to close without any hydraulic assistance.
In Alonso's case the actuator pulls the wing open with a simple link, much like the Lotus solution pictured above. However the mechanism allowed the flap to open too far, the airflow caught under the flap and flipped it over-centre to prevent the flap closing as it would usually do.
It was clear at the first stop that the flap easily pushed back into position, but on its first use on the out-lap the same problem occurred.
Unable to change the mechanism, Alonso was left without the system for the remainder of the race.
It appears that the mechanism failed to prevent the wing opening too far.
This 'end stop' might either be in the linkage or the hydraulic actuator itself.
Although this was a unique failure, Ferrari will need to review this problem ahead of the next race in Spain.