Alpha R2, needs to reconnect to graphic amplifier

My alienware alpha R2 tells me to reconnect my graphic amplifier when I boot . But when I do connect the graphic amplifier, it doesn't start and the same message appears on the screen and so I cannot start my computer

last time I shut the computer I click on the ondock button on the cable to ondock the amplifier

If you got any solution please respond as fast as you can.