My two big toe nails have completely cracked horizontally. One fell off half way and the other is yellow. They are ugly. What can I do about them?

Need to see a doctor. You need to be seen to determine if you have a fungus infection of the nail or a bacterial infection of the skin. Did you have an injury to the nail, etc?
Clip damaged part. Toenails will eventually grow out. Clip the damaged ends regularly and the cracks will grow out slowly. In meanwhile you can try a liquid bandage to keep the nail protected.
Depends on cause. Oftentimes nails yellow because there is something unhealthy (usually fungus) causing, this leads them to crack. Get them evaluated or if you consider toenail fungus, get treatment (topical or oral).
Have them. Evaluated for fungus and treated appropriately, .