A blow to the head must always be taken seriously. Even if the injured person seems fully recovered a short while afterwards, the blow may cause internal bleeding or swelling of brain tissue that sometimes only shows up days afterwards, and can be fatal.
Call a doctor at once if someone has received a blow to the head and they are unconscious for longer than a minute. Also call if they are unconscious for only a few seconds or don't lose consciousness, but show any of the following: a change in mental functioning e.g. confusion, sleepiness, slowed responsiveness; speech disturbance; weakness of a limb or the face; vision disturbance; nausea or vomiting. The person should rest for at least 24 hours after the injury, and a family member should keep an eye on them to check for any of the symptoms mentioned.