The door won't budge, or it only lifts a few inches before the opener strains and quits. Sometimes there's a loud bang from the garage - that's the spring itself snapping, not the opener motor. You might see a visible gap in the coil where it broke in half, usually near the stationary cone at one end.
On a heavier double door, you'll notice it the first time you try to lift it by hand with the opener disconnected - it feels like dead weight instead of moving with a counterbalance. A fraying cable, one that looks like it's unwinding rather than coiled tight, usually means the spring is already partway gone and the cable is about to follow it. None of this is a lubrication problem. WD-40 won't bring back a broken spring, and spraying the track won't stop a cable from snapping next.