No need for total loss, ala Hollywood. Rehab Congress talks about “any loss of memory for events immediately before or after the accident.”
Loss of memory for events before the accident is called retrograde amnesia, i.e., similar to the use of the term retroactive (something that relates back to a time prior to the event).
Loss of memory for events after the accident is called anterograde amnesia. This also includes problems with new learning.

The concussions that disable, are almost always more symptomatic at 24 hours, than at the 2-4 hour time frame when injured persons are evaluated in the emergency room. Brain injury symptoms escalate over the first 24 hours, because brain injury involves a cascade of events. It is critical that if you are still symptomatic the day after your injury, go back to the same Emergency Room, don’t wait for a doctors appointment. It is critical that the Emergency Room personnel see that the symptoms still persist or have gotten worse.