A broken cable is a torsion-spring problem even when the spring itself looks fine, and torsion springs store enough force to cause a serious injury if released wrong. This isn't a job we'd talk a homeowner through over the phone, and we wouldn't do it ourselves without the right bars and clamps.
What you can safely check yourself: whether the cable is frayed or fully separated, whether the door is sitting level or canked to one side, and whether the opener is straining against a stuck door rather than a broken one - unplug the opener if you're not sure, so it doesn't keep trying to force a door that can't move safely. Past that, it's a call for someone with the tools, not a Saturday project.