If the aircraft is at cruise altitude and speed, the wing will see a large loss of lift, and will become uncontrollable.
It used to be possible to deploy thrust reversers in Boeings. It was not feared as a problem, since early tests showed that accidental deployments were survivable and the plane would stay controllable. However, these tests were only performed at low speeds, since low-speed deployments were assumed to be the most critical (source).
But then in 1991 Lauda Air 004 crashed mid-flight in Thailand after an uncommanded thrust reverser deployment. After this crash, additional safety features such as mechanical positive locks were mandated to prevent thrust reverser deployment in flight.
I'm not sure about Airbus, but I guess all current aircraft have some kind of lock.
Because engines on modern aircraft are so closely attached to the wing, the thrust reverser exhaust flow is right in front of the wing. Therefore, the wing will see a lower flow speed which results in a loss of lift.