As you know, there are 2 cables for Coast 13. One is USB cable, another is HDMI cable.
Please plug the USB cable only into your computer. Will the indicator light on and show "No Signal" on the screen?
1.If "No Signal" doesn't appear, it's a hardware problem.
2.If "No signal" appears, it's the HDMI cable that isn't connected well.
To figure out what's wrong with the HDMI part:
1.You could connect the tablet with your computer via another HDMI-Mini HDMI cable.
If this helps, it means the HDMI cable is defective.
2.You could connect the tablet with another computer.
If this helps, it means the HDMI port of your previous computer is defective.
3.If all above doesn't help, the HDMI port of the tablet might be defective.