Well not being familiar with that model I had a quick look on the walkera web site and it looks like it is CCPM version, which means there are three servos around the main shaft. They are 'mixed' in the transmitter so that all three will move simultaneously to control the swash plate. For the pitch, all three should move in the same direction, pushing the swash plate either up or down. Without seeing a photo and presuming the servos all still move with TX inputs, I would take a stab that the bearing in the swash plate has popped out seperating the bottom and top of the swash plate. They can be popped back together, but if this is the problem you are better to very carefully put a few drops of CA on the bearing case to hold it into the plastic swash (after cleaning all excess grease residue from the plastic).

Or, just a thought, check you haven't coincidentally bumped the CCPM/norm switch on the back of the TX....