If the cable was charred at the end that connects to the phone, then it's most probably just the cable itself that shorted. However, any short circuit anywhere along the charging path, including the cable and charging adapter, can damage anything in that path. You can attempt to use the charging adapter again with another cable, but I advise you don't. If something is wrong with the charging adapter... something you might not readily see... it may damage whatever is plugged in at the end of the cable... like your phone.
I'm unclear what the cable was plugged into. When you state "USB charger outlet", I can't tell if you mean the charging adapter that came with the phone, or an outlet in your house that's set up for USB cables. That said, if it's the charging adapter that came with the phone, then it might be better to buy a new charging set up (cable and adapter), and destroy the old stuff, so they don't ever get used again. If it's a built-in USB outlet in your house, I would have someone check it out to make sure it's ok.
As an aside, you should never leave any charging/power adapter plugged in while it's not being used. Consider yourself lucky no fires got started.