Sleep problems or insomnia can be caused by many different ailments. One of the major causes for lack of sleep and chronic insomnia is stress and the ability to cope with it, therefore the person is constantly preoccupied with thoughts and worries in a destructive loop that never ends, including at night, thus preventing him from sleeping.
Another significant causes for insomnia are grief and trauma. The death of a relative, going through an accident or a scary experience can certainly explain your sudden lack of ability to fall asleep at night. If it has happened to you, a short period of insomnia is normal. Nonetheless, if the sleeplessness persists after a few weeks, you should definitely turn to a specialist for professional help, considering a medical or mental health treatment.