This is a copy of another answer I gave today:
Perhaps because your battery is dead.
Check your charger plug-in module with another USB device to see if it is still working.
If your plug-in is still working, check your cable for physical damage, such as a deep crimp in the insulation on the wire (which might indicate that a wire was broken inside), or a dinged-up male for the tablet.
There are 7 pins in the charger cable, buried down in the slots on that 30 pin connector, and the other 23 slots are empty. You might need a magifying glass and a bright light to see properly.
Even if you can't see any damage, a replacement cable is less than 5 bucks: buy a new one and see if it fixes your problem.
Remember if your battery is very discharged that it might take a while for the power-off battery logo to appear on the screen.