fired for locking restaurant doors upon being approached by official fron health dept. due to a water promblem with potentially hazardous water

You can be fired for any reason.  You can only sue if you can show there was an unlawful motive for your termination.  That would include unlawful discrimination and unlawful retaliation.  I see neither in these facts.  However, if the official from the health department gave you a direct, lawful order to close and you were fired for following the order, then you may have a claim.  Additionally, if the only way to protect the public or customer was locking the door (that may be difficult to prove), you might have a claim.