Could Extreme Stress Cause Long Term Amnesia?

My friend has suddenly lost memories of the past 4 years. She woke up one morning and didn t remember anything about the past 4 years. What could be the reason? She has been told about the 4 years that she can t remember. She can remember what she is being told. Her other skills like reading, writing, driving are intact! She recently lost her dad, about an year ago. She has been under a lot of stress. Her family took her to a general physician and psychiatrist but they said her memories will return on their own and ts due to stress. But could stress cause this long-term amnesia? What should be done? MRI?