Sometime between ages 40-55 (the average age in the U.S. is age 51), a woman begins to have irregular periods and then they eventually stop. Before and during this time, women may experience hot flashes, vaginal dryness, emotional changes, and a range of other symptoms with varying severity depending on the woman. 
You might be wondering: what happened to all of those 300,000 eggs that she had at birth? Why doesn't the cycle just keep going every month forever? 
Well, remember the stored eggs begin dying off at increasingly fast rates as the woman approaches menopause. This is in response to certain hormones sent by the brain.