Diagnosis
The clutch or the drive sprocket is not engaging. The primary cause is a failed clutch spring or a severely worn sprocket.
How to Fix It
Remove the side cover and bar. Inspect the clutch drum and sprocket. The sprocket should turn freely by hand when the engine is off. If it is seized, replace the sprocket. Next, check the clutch shoes. With the engine off, they should move outward slightly when you try to turn the drum. If they are stuck or the spring is broken, the clutch assembly must be replaced.
If That Didn't Work
The chain brake may be partially engaged due to a faulty mechanism. With the cover off, manually move the brake band back to its fully disengaged position and see if the drum turns freely.
Heads Up
Always wear gloves when handling the chain and sprocket. A worn sprocket will rapidly destroy a new chain. Match the replacement sprocket type (rim or spur) to what was originally installed.