What would happen if I am in coma without power of attorney?

Depends on state. There are different laws in each state. If you do not have a power of attorney or a living will which expresses your wishes should you be unable to answer yourself, usually the medical caregivers will look to your next of kin to make decisions for you.
Make wishes known. You can make your wishes known ahead of time by making a living will or filling out a 5 wishes document that lets everyone know what you want done. Also you should designate someone as your medical power of attorney who can speak on your behalf in medical decision making.