This can have multiple solutions so heres a few:
* The graphics port went bad, try hooking to a monitor known to work
**If the port is bad most desktops have several options for PCI slots to install a new graphics card
* The input on the monitor is wrong , for example if you're connecting HDMI bur the monitor is on VGA
* The power supply or other internal component could be bad
I would first try hooking it up to a 2nd monitor/TV.