My cameras were dirty (covered with dust) and I could not see the pictures clearly. So I cleaned the camera glasses with a rag soaked with glass cleaner. When I was cleaning the cameras the DVR started beeping. Then it started rebooting. Now my cameras are clean and I can see the pictures clearly. However, after rebooting the TV Screen indicates the message 'NO HDD' in Red Letters. I tried to change this but had no success. How do I fix this problem?