Quick answer
- The chain brake is probably engaged. Pull the front handguard back toward the top handle until it clicks. A handguard pushed forward locks the chain on purpose.
- An over-tensioned chain binds on the bar and won't move. Loosen the bar nuts and back off the tension until the chain pulls about 1/8 inch off the bar and slides by hand.
- Make sure the bar nuts aren't cranked down so hard they're pinching the bar and chain.
- Only suspect the clutch if the engine revs freely with the brake off and the chain still never moves.
Start with the chain brake
Nine times out of ten this is it, and it's free. The chain brake is that wide lever or handguard in front of the top handle. Pushed forward, it wraps a steel band around the clutch drum and locks the chain so it can't turn — that's the safety doing its job after a kickback or a bump.
Pull the handguard back toward the top handle until it clicks. That releases the band. People run the saw with the brake half-on without realizing it, hear the motor running fine, and assume the saw is broken. Reset the brake first, every time, before you go looking for anything mechanical.
Check the chain tension
If the brake is off and the chain still won't move, it's very likely tensioned too tight. A chain cranked down hard presses into the bar rails and simply can't slide.
Loosen the bar nuts a little, back off the tension screw, and re-set it: the chain should sit snug in the groove but you should be able to pull it about 1/8 inch off the bar and slide it freely around by hand with a gloved hand. Then re-tighten the bar nuts. While you're there, make sure the side cover is seated properly and the bar nuts aren't so tight they're clamping the bar against the chain.
Clear out packed debris
Sawdust, bar oil, and wood chips pack into the bar groove and around the clutch drum and sprocket, and that gunk can bind the chain. Pull the side cover, clean the groove with a thin tool or a stiff wire, and clear the debris from around the sprocket. It's a five-minute job that fixes a chain that drags or won't turn freely.
On a battery saw, check the obvious too
Cordless chainsaws add a couple of free checks. Confirm the trigger lockout is being pressed and that the chain brake didn't trip from the saw being knocked around in transport. A battery that's hot or nearly dead can also leave the saw running weak, so make sure you're on a charged, cool pack.
When it really is the clutch
If the brake is off, the tension is right, the groove is clean, and the engine revs up but the chain never moves, then the centrifugal clutch is worn out. The clutch shoes or springs are supposed to grab the drum as engine speed rises and drive the chain; when the friction material is worn, the engine spins but nothing transfers to the chain. That's a replaceable assembly matched to your saw.
Don't start here, though. Most "chain won't spin" cases are an engaged brake or an over-tight chain, and swapping a perfectly good clutch is a waste of an afternoon and a part. Work the free checks first — the clutch is the last suspect, not the first.