3 year old female rat lost weight?

Ive got a 3 year old female rat who has been living without a cagemate for about 5 months and shes not looking too well. She can no longer walk up the ramps to get to her food dish/water bottle, and she feels a little "bony". her fur looks fine, but again, she looks a little bony, and feels a little "dead", cant really explain it very well. I know that shes 3 years old, quite old for a female rat, but I dont want to do nothing and feel bad when something happens to her because of my inaction. Ive moved all her food and water down to the bed of the cage, and moved her tube home so she can get in and out of it easier, but she just lies out in the open where I left her. Is there anything I can possibly do for her?