A broken cable on a torsion-spring door means the spring is still loaded, and that's the part that hurts people, not the cable itself. We don't recommend homeowners open the door or touch the drums once a cable has come off - even a door that looks like it's resting fine can drop or twist if the remaining side takes the full load.
Snoqualmie's median household income here runs around $197,531, well above the wider service area's $119,013, and we mention that only because it tends to mean newer doors with heavier insulated panels - more weight on the spring system than a lighter single-layer door from decades ago. That's more force stored in the spring, not less, which is exactly why a DIY cable swap on a door this age is a worse idea here than on an old lightweight door.
What you can safely do: unplug the opener so it doesn't try to cycle, and leave the door alone until we get there.