why solid nacl is bad conductor andin aquoessolution nacl is good conductor

Solid NaCl is bad conductor because in solid state NaCl has no free electron but if we dissolve NaCl in water then it dissocate into ions which cause it conduct electricity in aquoes solution.
 

See the NaCl molecule is an ionic compound ,so,due to the strong electrostatic force between the Na(positive ion) and Cl(negative ion) as a result their is no free electrons in the compound in solid state but in liquid state(molten or aqueous),I mean when dissolved in water the water molecules break the NaCl molecules into ions due to the uneven charge distribution and in molten state when the compound is strongly heated the electrostatic force of attraction between them reduces,hence breaking it into Na+ and Cl-,that's why NaCl (solid ) is not an electrolyte