＊Check the adaptor of camera whether to see if it is well-connected.
＊Check the cable for connecting video input/output in the back panel of DVR.
＊Insert the video source directly into the display device and check if they are causing the problem.
＊Check the brightness of the picture and bring it back to its original default setting.
＊No video input signal or too weak.
＊Display settings in the preview set to be closed.
＊The hardware of DVR is defective.