Monitor Not Displaying

Hello,
Last night I unpuged the power supply of my CPU and then again plunged I checked that everything was perfectly connected but when I started Computer, at first, the Monitor was not displaying contents I connected Monitor's cable properly than It started displaying but it gave error "Non-System or Disk error" my friend plunged the internal wires properly but than again Monitor stopped displaying. It's yellow light is on(not Blinking) but still not displaying anything.