Grandchild can't walk or talk at 4?

What illnesses or disease can effect a child's ability to walk or speak. I have a 4 yr old granddaughter who took her fist step when she was 1 yr old and 3 weeks later she could stand up and she can't control her body. this affected her eyes, mobility, and speech. She can crawl,she can stand as long as she is holding onto something, she can see and understand but can not walk or talk. She is 4 years old and quite tall too tall to be crawling.Her feet are narrow and flat as though the muscle has been changed. I am desperate for information and solution. If someone can help me please respond.