DIAGNOSIS The drive sprocket (the toothed gear that moves the chain) has likely stripped out. It's a common wear point, especially if the chain was too tight or the door spring is too strong, putting extra load on the plastic gear.
FIX You'll need to replace the drive sprocket assembly. Unplug the opener. Remove the light cover and lens to access the head unit. You'll see the chain and the sprocket it rides on. The sprocket is part of a gear assembly that includes a clutch. The replacement part is often called the "gear and sprocket assembly" or "drive gear kit" for B4545 models. Follow a video guide to swap it out—it involves removing a snap ring and transferring the motor coupling.
IF THAT DIDN'T WORK If the new sprocket assembly still doesn't move the chain, the issue could be with the logic board not sending power to the motor properly, but the stripped gear is by far the most common cause of this specific symptom.
HEADS UP While you're in there, check the chain tension. It should have about 1/4 to 1/2 inch of slack when you push up on it in the middle of its horizontal run. A too-tight chain is what usually kills the sprocket in the first place.