Can anything be done when there is only one witness to a crime?
Two witnesses are required to establish legal truth in order for civil government to condemn (Deut. 19:15ff). This doesn't prevent anyone from acting (within the law) upon a report or their own witnessed evidence. A single witness should absolutely report a crime to the judge. The judge can then call for additional witnesses. The first witness may not be aware of other witnesses. Even if the murderer isn't convicted (because there is only one witness), no one is required to "carry on as if it hadn't happened." The single witness might be utterly trusted by everyone in the community, such that they are confident in the guilt of the murderer. Maybe they can choose not to do business with the murderer. The community just isn't allowed to use force (e.g. the death penalty) against him.