1) Hens are too young to lay:
Many chickens look full grown before they actually are. Depending on the breed, your hens may not start laying till they're almost 8 months old! I've heard of Silkies that didn't lay till they were 9 months old. While most hens will start laying between 4 & 6 months old, it might just be a little too early for your hen to lay. Hang in there a few months, she should start laying in time.
2) hens are too old to lay:
Egg production in most hens slows down after age two. As they age from there, the amount they lay can decrease each year. My oldest hen is 8 years old and lays about 5 eggs a week for maybe 2 months in summer. Some will lay more than this, and some will lay much less. This can also depend on breed.
3) A hen is broody:
When a hen goes broody she starts sitting on eggs to hatch them, but she stops laying more eggs. Even if you take away the eggs she's sitting on, the hen won't start laying again as long as she's broody. It's a hormonal thing, she can't just change her mind to not sit. 
4) Insufficient diet to produce eggs:
Laying hens need a healthy, balanced diet. Too many treats or not enough vitamins, minerals or protein can cause them to stop laying eggs. Make sure your hens have access to good quality feed and plenty of clean water.
5) Molting hens don't lay eggs:
Chickens generally tend to molt in fall, though molting can happen at any time of year. They will experience a pattern of feather loss along with a cease in egg production. Molting generally lasts about 2 months as their feathers fall out and grow back in. Hens will start laying again when their molt is done.
6) Illness:
Illness can take its toll on a laying hen. As her body is fighting to get better, egg production might stop. She should start laying again shortly after she gets better.
7) Parasites:
Becoming infested with external or internal parasites can cause a hen to stop laying. Red mites feed on blood which can cause anemia. Check between feathers and around vent for signs of mites and lice. Internal parasites like worms can cause a decline in nutrient absorption.
It can be difficult to tell if a chicken has internal parasites. Diarrhea, eating more, losing weight and a stop to laying can all be symptoms of worms.
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8) Trauma:
Injury, attack or other trauma can cause a hen to stop laying eggs. Once healed up, an injured hen will resume laying.
9) Drama:
Yes, chickens have drama too! The pecking order is very real! A hen that is bullied or picked on may stop laying. Watch to make sure that there isn't any abnormally mean behaviors in your flock. A move, new coop, or new flock mate may also cause hens to stop laying.
A predator attack can also cause the survivors to stop laying for awhile. Give it some time for the drama to blow over and the girls should be back to laying eggs in no time!
10) Weather:
Extremely hot or cold weather can throw your hens off their laying schedule. Every summer when it gets brutally hot my girls go from giving me a dozen eggs a day in the big coop, down to 1 or 2. Once the days cool down just a bit, all the hens go back to full time laying.
