That’s an excellent question. There can be many reasons why people might not be able to walk. The reason behind that is very important when considering how to help.
For example, one such reason is because of an injury of the spinal cord (which is the topic of my work). When someone injures their spinal cord then depending on the severity of the injury different outcomes might happen – from losing some ability to move and feel the body, up to being completely paralysed from the place of the injury and bellow. This is because the signals that come from the brain (to move the muscles) and to the brain (to get all the sensation) cannot go through the area of the injury.
In the project I work, our participants have spinal cord injuries that mean they are completely paralysed bellow the chest – they cannot feel or move any part of their lower body. When this happens the muscles, but also the bones, start getting weaker because their not used anymore.
Before we start our experiments (which is doing standing exercises with our participants), the participants must train their muscles so they are strong enough to stand. To train their muscles we use a device that can electrically stimulate the nerves that contract the muscle; this means that muscles can be contracted without the need for a signal to come from the brain. After they strengthen their leg muscles then they can stand and exercise (again using the same technique of electrical stimulation).
Some other researchers have managed to use similar techniques to activate the muscles of each leg at the right time to produce walking. This is a great achievement, but it is still very slow, expensive, and impractical. However, in the future this technique could be greatly improved.
To go back to your question:
For some health conditions (like a spinal cord injury), strengthening the muscles is a possible step towards walking again. For some other conditions (like multiple sclerosis) this unfortunately is not enough, as the damage in the nerves that carry the signals is much wider and ongoing.