A muscle pulls by sliding filaments: thin actin ropes anchored to the Z-discs are dragged inward by thick myosin filaments floating in the middle. Each little myosin head is a cross-bridge — but it can only pull where it actually overlaps an actin rope.
In 1966 Gordon, Huxley and Julian measured this on single frog fibres and found force follows the geometry exactly. There is a plateau of maximal force where every head has actin to grab. Stretch beyond it and the overlap — and the force — falls away in a straight line, hitting zero when the filaments no longer touch.
Shorten too far and the opposite trouble begins: the thin filaments run past each other and collide, then the thick filament jams against the Z-discs. Both extremes kill tension. Layered on top, the giant spring protein titin resists a stretched muscle passively — which is why a pulled muscle still pushes back even when it can generate no active force at all.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.