How would you describe the situation to police if your neighbors had called them and your left arm was missing?

This is certainly a complex matter! First, let's consider a few possibilities.

Is your left arm being missing a new occurrence? Perhaps you lost your arm quite a while ago. If your arm being missing is not a new occurrence, we can disregard this information.

If your arm being missing is a new occurrence, you should report this to the police. If it is a prosthetic arm which has gone missing, you could report this to the police as an instance of theft. If your physical, bodily arm has been removed and is now missing, the police will likely treat this as a much more serious case. Your possible injuries should be treated and accounted for, and your neighbors should alert the police or emergency services to the fact that you need medical attention.

That brings me to the next point to consider-- why are your neighbors calling the police? Is it because your arm is missing? If so, they are very kind to have called emergency services for you. Be sure to tell the police that it was your neighbors who made the call and offer as much information about your situation as possible. Your neighbors may have additional information depending on your situation. For example, if they called the police because you were found unconscious, your neighbors should share as much information as possible about how, when, and where you were found. 

If your neighbors have called the police for a matter entirely unrelated to your missing arm, just be sure to share any information you may have regarding the situation at hand. Whether it's to do with your missing arm or not, just try to answer questions as best as you can and offer any information which might be helpful in resolving the situation.