A garage door cable runs under real tension, wound tight against a spring that's doing the actual lifting. Even with the door down, the drum and cable are still loaded. Cutting a frayed cable loose or trying to rewind one onto the drum without releasing that tension first is how people lose fingers or take a face full of spring - this isn't a job for a Saturday with the wrong tools.
If you're comfortable and it's just cosmetic - tightening a loose bracket, oiling a roller - that's fine to handle yourself. Cable work isn't in that category. We're straight about that limit: we'd rather tell you no than show up to a job that was made worse by a first attempt. If your door's stuck down with a broken cable, the safest move is to leave it down, keep the opener off, and call rather than force it.