DIAGNOSIS The centrifugal clutch is either stuck engaged (rare) or, more likely, the drive sprocket or clutch drum is seized up from lack of lubrication or grime buildup.
FIX First, check the obvious: make sure the chain brake is disengaged. If that's not it, remove the side panel and bar. Look at the clutch drum (the sprocket the chain wraps around). Try turning it by hand—it should spin freely when the engine is off. If it's stuck, you need to clean the whole clutch assembly. Gunk from the bar oil and sawdust can glue it solid.
IF THAT DIDN'T WORK The clutch springs themselves might be broken or the clutch shoes might be worn out. If the drum spins but the chain still doesn't move when you rev the saw, the clutch isn't engaging. That usually means replacement.
HEADS UP Never run the saw without the bar and chain on, trying to 'free' the clutch—you can over-rev and damage the engine. Always do clutch work with the spark plug disconnected.