All video monitors today have a minimal input signal level.. and if below that, will go into an energy saving mode (yellow LED indicator). Many video cards do loose signal strength over time.. due to aging capacitors or low voltage. Testing the monitor by itself on several sources should reveal its usefulness. But flat-screen monitors have a shortfall.. the backlight system. Tiny fluorescent tubes in the bezel provide the illumination. They have a similar life span as common lighting fluorescents. If no image yet a green "active" LED indication, try shining a bright light at the monitor face, while a motion image is displayed. If you note faint images or movement, then the backlight system has failed. Could be either the age of the tubes, or the inverter has failed. Either way, very intense labor to repair.
F@H.. to solve mankind's maladies.. in our lifetimes!