Cable work is spring-adjacent work, and that's where the line sits. The cable runs under tension from the spring system, and disconnecting one without releasing that tension the right way is how people get hurt - a stretched extension spring letting go near your hands isn't a small mistake. We don't recommend doing this one yourself, full stop, even if the hardware store cable looks like a cheap fix.
What you can check safely: whether the cable is visibly frayed near the drum or bottom bracket, whether the door is hanging crooked, and whether the opener still runs (it usually does - a dead cable doesn't kill the motor, it just means the door can't lift). That's useful information to have ready when you call, but the repair itself isn't a parts-store Saturday project on this kind of door.