Sometimes if you concentrate too hard on getting to sleep, it has the opposite effect. Like Alex D said, if this is a chronic problem, you should talk to a doctor about it. If not, don't try so hard! Use relaxation techniques to clear your mind, and sleep will come. Maybe your sleep schedule got thrown off, or maybe you slept in too long that morning. Regular sleep schedules are important not only for dreaming, but for good, restful sleep for your body's sake.