Making and storing memories is a complex process. It involves many regions of the brain, including the frontal, temporal and parietal lobes. Damage or disease in these areas can result in varying degrees of memory loss.
Here's a good example of how memory loss can happen. For short-term memory to become long-term memory, it must go through a process known as consolidation. During consolidation, short-term memory is repeatedly activated - so much so that certain chemical and physical changes occur in the brain, permanently embedding the memory for long-term access. If, during this repeated activation, something interrupts the process - let's say a concussion or other brain trauma - then short-term memory cannot be consolidated. Memories can't be stored for long-term access. This may be what is going on in anterograde amnesia.
It is believed that consolidation takes place in the hippocampi, parts of the brain that are located in the temporal-lobe region of the brain. Medical research indicates that it is the frontal and temporal lobes that are most often damaged during head injury. This is why many people who suffer severe head trauma or brain injury experience anterograde amnesia. If the hippocampi are damaged, the amnesiac will be able to recall older memories, but won't be able to make any new ones.
So, yes, it is possible to suffer memory loss - amnesia - as the result of a blow to the head. But, the head trauma must be severe. For a bonk on the head to result in amnesia, the bonk would have to be so incredibly hard that it would cause severe swelling around the temporal lobe and/or injury to this region of the brain. Obviously, another blow to the head would merely result in more head trauma and would most likely exacerbate the problem!