Medications, usually. Many medications have the side effect of reducing salivation. If you are taking any meds, ask your dentist or physician if they can cause this problem. Beside medications, dry mouth can also be caused by recreational drug use, radiation to the head or neck for cancer treatment, coffee/tea, anxiety and others that don't come to my mind at this moment.
Different causes. It is my experience that the most common side effect associated with drugs in dry mouth. So first you would have to check if you are taking anything, including cold medicines and other over the counter drugs. There are also some diseases that have dry mouth as a concern (like soldgerns). Please check with your dentist or internist asap because saliva is so important for overall health of mouth.