Why did my lights go out when I accidentally dropped a charger plug on a power cord with empty sockets?
That small bright spark you saw was probably the prongs of the charger, with the weight of the charger behind it, penetrating through the extension cord insulation and hitting one or both of the extension cord's conductors.
You should carefully inspect the extension cord for the damage.  It potentially might be a shock hazard if it were to get wet at all, so its probably best to either wrap with electrician tape to protect casual contact if you intend to continue  to use the extension cord.
Most of the time the electrical tape will be sufficient for guarding against a shock hazard but realistically, if you're in doubt, get a new one.