Hello,
I am afraid you will need to take your dog to your vet for an examination. I think it is unlikely she has been involved in an injury to her legs. From what you describe it is more likely that she either has some problems with the nerve supply to her legs (this might be caused by a disc problem in her back (like a 'slipped disc' in people) or due to problems with the nerve supply in her legs (this can be an old age related change).
It may also be that she has arthritis in her hips (also common in older large breed dogs) and if she has been stiff in her legs before now this may be more likely.
In the meantime I would help her to her feet when she needs to get up but then encourage her to walk for herself out to the yard etc. remember to make sure she can easily access her water.
Katharine
Dog Veterinarian
I have been qualified for 20 years and have additional quals in small animal medicine/imaging