It's in a self Diagnostic test mode.
check to see if the colors are moving in a square arond the monitor and if there is any writing on the screen.
Make sure the wire from the monitor to the computer are plugged in properly- Reseat them. Take them out and place them gently back in. The wires should be snug not tight. they should not fall out by themselves.
check to see if you have a secondary video output from your monitor and if so attempt that connection as well. The computer will normally self recognize an output and a modern monitor will under normal circumstances project an input.