What breed of dog? How old?
Do you mean she hasn't been anywhere but in a fenced yard? Or did she have her tail closed in the fence?
She could have some type of inflammation to the tail, sometimes we do not know the causes of these issues, sometimes we know of a trauma, and sometimes even over exertion will cause this. Since she is 10 1/2, I would assume she is not too active. But, if she went out for a long walk, or was running around more than usual, that could cause this type of a problem. It is called cold tail, or dead tail. It usually goes away on it's own after a week or two. If it causes much discomfort, you can give them aspirin or some other type of pain reliever (prescribed by your vet). Since she isn't painful, she may not need anything. Here is an article about dead tail: http://www.dockdogs.com/index2.php?option=com_content&do_pdf=1&id=204
Another possibility is that she has something bothering her under her tail in the anal area. Anal gland impactions and infections can cause a dog to hold their tail down over the anus to protect the area.
A back injury can also cause the dog not to be able to lift the tail or wag the tail, as it will interfere with the nerves going to the tail.
You will need a vet to help you figure out what the cause is. If you cannot get her to the vet you can always wait and see if the tail improves. Make sure to look around the anal area to see if you can see any sores, swollen areas, or redness. Touch around the anus to see if it is painful. If you find anything abnormal, you should try to get her in to see the vet.
Let me know if you have more questions.