Is there a display on another monitor? If so, if your graphics card has two display ports, plug in the Viewsonic and the other monitor at the same time. Boot up your computer and go to your display settings to see if your computer detects the Viewsonic monitor. In your display settings, you should see two monitors labeled "1" and "2." Try to extend your desktop display to both monitors and see if the Viewsonic pops up a display.
Confirmed its the monitors problem cuz i tried with other lcd monitor and pc works , i also tried plugging the problematic monitor to other pc's and the problem persist.