There are many causes of hair loss. Getting to the root of the problem is tricky because often the trigger for hair loss is something that occurred in the last few weeks or months. For instance, if 2 or 3 months ago you experienced a fever or high stress, this could have produced the hair loss you are seeing now. Crash diets, an infection or serious health problems can also lead to hair loss.
Other factors that may cause hair to fall out are medications, insufficient or too much of certain nutrients, hormonal problems, ringworm, and various medical conditions. An underactive thyroid gland is a common culprit in hair loss, and one with an easy fix. A doctor can sort this out with you.
Once the cause of the problem is addressed, your hair should stop falling out and will eventually grow back. It may take many months for this to turn around. Unfortunately, there are some instances in which hair loss is permanent, as in cases of genetic baldness. However, there is help for hair loss, such as minoxidil (Rogaine), which is available by prescription.