To connect a Viewsonic monitor to a PC, fix one end of the VGA signal cable on the monitor and the other to the output port on the PC. If the input port in the ViewSonic monitor is not identical to the output port on the PC, then you should connect with an adapter.
When a ViewSonic monitor is not working, it is often a power problem. Disconnect all cables, including the cable between the computer and the monitor. Inspect them for any damage and if they are damaged or older, they should be replaced. You may also need to restart the computer fully to get the monitor working again.