DIAGNOSIS The brushes are worn down to the nub or a brush spring is stuck. The GBH2-28L has user-replaceable carbon brushes accessed from the sides of the motor housing.
FIX Unplug tool. Use a flathead screwdriver to pop out the two rectangular brush caps on opposite sides of the motor housing. Pull out the old carbon brushes. Check they are longer than 5mm. If not, replace with Bosch part number 1 607 001 007. Ensure the new brushes slide in freely and the springs push them against the armature.
IF THAT DIDN'T WORK The armature commutator could be dirty or burned from sparking with worn brushes. You'll need to disassemble the motor to clean the copper segments with fine sandpaper.
HEADS UP Running the tool with completely worn brushes will score and permanently damage the armature. That's a much more expensive fix than a $15 set of brushes.