Mother has lost her mobility in her legs and we don't know why. What could it be?

Diabetic neuropathy is a frequent diagnosis when repeated falls are a problem. But another common problem is "spinal stenosis" in the elderly, which has no relationship to diabetes.

Arthritic changes can cause bony growths to press against the spinal cord interrupting messages from the brain. The legs themselves are not "weak", but may simply fail to receive nerve impulses from the brain that allow them to properly support the body. It can be puzzling because there may be no significant pain, only some periodic loss of sensation and/or repeated sudden collapses without warning.

Diagnosis is by an MRI and treatment by a neurosurgeon.
The cure is laminectomy, a spinal surgery that is not nearly as painful nor disabling (in my own experience) as a joint replacement. A couple of weeks rehab (more or less) and physical therapy is required following the procedure. My own results (and others I'm familiar with) have been very good. I'm a couple of years post-op ( 80 yrs old now) and have not fallen in 2 yrs! My mother also had to have this surgery, so there may be a hereditary component. She also recovered nicely and this was quite a number of years ago.

My brother had terrible neuropathy from diabetes. He died in his sixties. He used a cane at first, then a walker.

I know a woman who has Down’s syndrome that suddenly stopped walking when she was diagnosed with dementia. It’s strange. She just turned 51. She used to walk fine, even dance to music, then completely stopped walking. The hospice nurse said they have seen this before with other patients. She is now in a wheelchair.

I am so sorry. Best wishes to you and your family.

I agree with BBS about the possibility of neuropathy. Has any MD given you a supposition as to what has been going on. In the latter stages of dementia it is not uncommon to elders to stop walking, stop eating, and etc. If things go on long term with a healthy heart, lungs and kidneys elders can end emaciated and in a fetal position in bed, all but non-responsive. All the tests have been done at this point, and your MD is the best one to "guess" at what is going on. People always expect medicine to "have answers" and often enough it is, as my oncologist told me "anything but an exact science. At this point we are just becoming smart enough to know what we don't know.

Diabetics develop something called diabetic neuropathy, which causes pain in the extremities. If she lacks strength in her legs, and has hypertension and elevated cholesterol plus the diabetes, she may have something called peripheral vascular disease, which means she is not getting sufficient blood flow to the legs and feet. Medication can help alleviate some of the symptoms but nothing is going to reverse the process at her age.

And inactivity, like sitting all day, allows the leg muscles to atrophy and weaken.

If she's fallen 5 times in one week, that is a bad sign. 50% of elderly who start to fall die within the next two years. That's a statistic I learned when I worked for a life and disability insurance firm for 40+ years. Living alone is no longer an option for her. She probably needs a nursing home, since assisted living facilities cannot keep residents who fall continually.