My goldfish can't open its mouth.?

i would anaesthetise it with clove oil and gently open its mouth to see if i can see whats the problem, if it cannot be opened then simply overdose the clove oil to finish the fish off humanely. research clove oil and goldfish and it should bring up all the details you need, also www.aquariacentral.com is an awesome site with tons of very helpful extremely knowledgable members always happy to help.