DIAGNOSIS The hammer mechanism is disengaged. The mode selector is likely stuck in 'Drill Only' position, or the internal hammer fork is broken.
FIX First, ensure the mode selector dial is fully clicked into the 'Hammer Drill' position. If it feels loose, the problem is internal. You must open the gear housing. Check the white nylon fork that moves with the selector. If it's cracked, replace it (part number 1 607 001 005).
IF THAT DIDN'T WORK The wobble plate or drive pins that engage the hammer cylinder could be sheared off. This requires replacing the entire crank assembly, a major repair.
HEADS UP Using the tool in 'Rotation Stop' mode for chiseling without the hammer function is normal. If you're in 'Hammer Drill' and get no impact, stop. Forcing it will wreck the gearbox.