Video 
The
monitor cable plugs into a port on the video card with places for 15 pins, but there are not 15 pins on the cable connector. The sides of the plug are sloped so there is only one way to insert the plug.
Digital monitors have a different connector
without pins and require a digital video
card.
The video connector seems to be easier to knock off than the other connectors. There are screws on either side to fasten it down. Keeping it fastened down will protect the pins in the plug from getting bent. It is easy to bend the pins by pushing too hard when the pins and holes are not quite lined up. You may think that you have a good connection. If the color is not right on the monitor after you've connected it back up, you have probably bent the pin that carries the instructions for red. It seems to be the one bent most often.
You can straighten a pin that is out of alignment by carefully using a small flat blade of a knife or screwdriver to move the pin back in place. If it's really kinked, take hold of the pin with needle-nosed pliers and gently straighten it. Be VERY gentle. You don't want to get into the problem of replacing the video plug.