If you have a Kindle Fire 7" 1st gen then your battery is probably dead and the USB power port has come loose. The first Kindle Fires had a poorly designed USB power port which could easily come loose from the motherboard. When this happened the device could no longer be charged and when the battery ran down then the device would go dark and was essentially "bricked."
The solution is to open up the device and replace (or reattach) the micro USB power port, but this requires technical skills including knowledge of electronics and soldering.
Good luck with this!