Anytime you lose consciousness, from a traumatic head injury, is an emergency. If this happens to you (in general) - get to an emergency room. Call an ambulance right away. If you find someone, or yourself, in this condition - check the eyes of the victim. If one is dilated - that a good indication that subdural hematoma is taking place - bleeding in the brain - and that's grounds to expedite the ambulance - lights and siren.

Fortunately the remedy for this is simple, but must be performed by a physician.

I trained as an EMT. A full-on course developed from battlefield-medicine in Vietnam to the present day. Think about taking the course. Even if you don't become an ambulance-jockey, the training can save a life.