[F]

My wife is 49 years old. She have had her monthly cycles regularly and never have had any irregularity until eight months ago. She missed her one cycle but had her normal cycle the following month. Then she missed her cycles for three months. She saw her gynecologist who told her after blood tests that she was at least several years away from menopause. Since last five months she had no cycles. As I am a male, I do not understand or know about the women health that much, I like to seek advice here.

• Can the monthly periods stop all of a sudden or is it a gradual process?
  
  What could be the reason for such an abrupt stoppage?
  
  What could be the associated health risks?

Any advice or suggestion will be appreciated.

PS. We are happily married since last 29 years and have four children.

As long as her doctor said she is not near menopause, did he happen to mention why she'd stop menstrating?

Menstration can stop suddenly, much as you described. Without seeing her doctor, I can't give specific advice, but in general reasons that would cause lack of menstration could be:
1. Stress
2. Diet changes/weight loss/weight gain
3. Illness
4. Anemia, etc.
5. Menopause/pre-menopause

There is no health risk to her if she does not menstrate. However, lack of menstration could be a symptom of something such as another illness. I would recommend she check in with her doctor again to inquire.