Summarize this article:

It's possible that you are the cause of a frightened dog's anxiety. But it could also be that the dog is bothered by something in his immediate environment, and your presence is not a factor in his fear. Always assume that you might be the stimulus that is causing a dog to feel frightened or nervous, especially if it is not a pet you are familiar with. If it's a dog you know and encounter regularly, you may observe multiple situations or stimuli that seem to upset the dog. Understanding what triggers this response can help you avoid those triggers, or expose the dog to those triggers in a controlled environment.  If you know the dog, scan your surroundings for things that might be upsetting the animal. Things that we take for granted, like a certain sound, a foreign object in the yard or house, or a new and unfamiliar odor, could cause a dog to act fearful or nervous.  If the dog belongs to you or someone you know and you suspect that an environmental change is the reason the dog is upset (a vacuum cleaner, for example, or a new piece of furniture), expose the dog to that new element in short, controlled sessions. Allow the dog to become familiar with that object and to recognize, in his own time, that the object is not a threat.  Examine the dog for injuries from a safe distance. Do not force close contact. Take note of whether the dog is limping or hiding one side from you, keeping his ears flat against his head, excessively licking an injured area, or whimpering. Approaching carefully, and knowing when to back away, are crucial for a successful interaction with a shy or frightened dog. Remember that if the dog does not know you, he may perceive you as a threat.

Summary:
Determine what is upsetting the dog. Observe what may be frightening the dog. Do not further frighten the dog.