It is not uncommon for strings, hair or other long fibers to get wrapped up in a brush bar. This can cause the brush bar to slow or stop. Examine the brush roll for long fibers that may be caught in the brush and remove them with a pair of scissors after turning off and unplugging the Dyson vacuum.
If the plunger on the sides of the brush bar is extended, loosen the plastic fasteners on each side of the sole plate and lift it off. Pull out each side of the brush bar and examine it for other obstructions that may be preventing the bar from turning when it's inside the sole plate.
There is a set of metal shafts where the brush bar is positioned during normal use. Look inside the metal shafts for items that may have gotten suctioned up by the vacuum cleaner and caught. This can damage the brush bar or prevent it from turning. Clean the shafts as needed.
There is a duct that is behind the brush bar where the unit is connected to the hose. Obstructions are also common in this area, so examine closely and remove anything that may be causing the problem. If you replace the brush bar, position the arrows on the brush toward the back of the vacuum before installing.