One of my tooth gum has receded and the bottom root is sensitive to touch. What can I do to fix this? How did this happen? Will the sensitivity heal?

Recession. Can be caused by improper brushing technique or toothbrush, poorly positioned tooth, infection (gum disease), occlusal trauma, gum-line decay, and other reasons. You MUST see your Dentist before this gets worse. Temporarily use desensitizing toothpaste (on a very soft toothbrush) and fluoride oral rinse.