Things to try:
1. Check the cable
2. Check the monitor on another PC
3. Install a video card (if it now has video from the motherboard) or another video card (if the current one broke).
I have a Lenovo think centre desktop and it worked fine until I unplugged it (I shut off pc first) it only has a VGA port and it is connected to my LG monitor, everytime I turn on my pc it just says no signal hellpppp