Will you come back and let us know she's ok? 
I hope it's simply one of those teenage hormone things, but there are other possibilities besides the two discussed on this thread. From here:
"When should I be concerned?
Your daughter should be checked out by a doctor such as an adolescent medicine specialist or pediatric gynecologist with expertise in figuring out menstrual problems in teens. So if your daughter hasn?t started her period by age 15 or three years after starting breast development or she has had her period before, but stops having them, or even if she?s just late, it?s always a good idea to check in with a healthcare professional."
And
"What complications is my daughter at risk for?
The most important thing is to figure out the cause of the late periods:
She may be fine and just a little late with regulating her periods.
She may a hormone imbalance called polycystic ovary syndrome and needs to be checked for cholesterol and diabetes.
She may have an ovary or thyroid problem and need hormones.
Or she may have low estrogen levels from excessive exercise or low weight, and may be at risk for low bone mass.
Without enough estrogen, not only do menstrual cycles stop, but also the body is prevented from being able to absorb calcium to build strong bones. This can result in low bone mass. Very little bone mass is added after you are 20 years old, which means that the teen years are very important for getting the right amount of bone for your lifetime.
Everyone (teen girls especially) needs the right balance of exercise, body weight, calcium intake, vitamin D, and estrogen levels to have healthy bones. Make sure that your daughter takes in 1200-1500 mg of calcium and vitamin D (600 units/day)."
Please do make sure she is checked out. From what you have said it's been longer than 45 days. It could be the first symptom of a whole long list of other problems that it is good to find early. It is a remote possibility, of course, but definitely worth excluding by a quick and simple gp check . After all, if it was an adult MN user then we would tell them to get checked, so please do the same for your dd.