When a child's departure unleashes overwhelming sadness, treatment is definitely needed. Discuss your feelings with your general practitioner as soon as possible. You may need antidepressants, and you almost certainly could use some counseling to get your feelings into perspective.
Meanwhile, look to your friends for support and be kind to yourself. There are practical things to help you feel better. For instance:
Time and energy that you directed toward your child can now be spent on different areas of your life. This might be an opportune time to explore or return to hobbies, leisure activities or career pursuits.
This also marks a time to adjust to your new role in your child's life as well as changes in your identity as a parent. Your relationship with your child may become more peerlike, and you will have to get used to giving your children more privacy.
Many suggest preparing for an empty nest while your children are still living with you. Develop friendships, hobbies, career, and educational opportunities. Make plans with the family while everyone is still under the same roof, so you don't regret lost opportunities: Plan family vacations, enjoy long talks, take time off from work. And make specific plans for the extra money, time, and space that will become available when children are no longer dependent on you and living at home.