I've been having stiff neck problem for almost a week now. I can't turn my head properly. Usually it took only a day or two. I checked my cholesterol.?

Torticolis. Cholesterol shouldn't have anything to do with a stiff neck. It sounds as though you have strained the neck muscles leaving them in spasm. Time and anti-inflammatory medications such as tylenol (acetaminophen) or Motrin may be useful in relaxing the tissue to normal. Failing that, you may want to look into osteopathic or chiropractic manipulation. Daily exercise can help avoid this problem before it occurs.
Torticollis. Torticollis is the fancy medical term for a stiff neck. Depending on the cause, this is often very well treated by osteopathic manipulation in the hands of an experience practitioner. I would suggest finding a do in your area who performs omm and see what they can do for you.
Sleep wrong? Assuming there was no trauma nor fever; most 'stiff necks' are from the result of poor quality of sleep. Check your pillow and mattress at home and replace if needed. Try a warm compress; or a warm/hot shower may be what the doctor ordered. From the osteopathic side you may also have an elevated first cervical rib which is easily treated. If the problem persists see your doctor.