An inability to form semantic memories would most likely result from damage to the:

Hippocampus. This is a type of amnesia called anterograde amnesia which manifest as inability to form new memories after the brain was damage. This can also lead to impairment of remembering new facts and events.  The area in the brain that is associated learning and memory is the hippocampus. Any damage in this area whether due to hypoxia, stroke, or any form of pathology can cause anterograde amnesia. This is a part of the limbic system that plays a role in consolidation of short term to long term memory and spatial memory that allows us to navigate. This area is located under the cerebral cortex.