The battery connector in the Nexus 7 (2012 model) is notoriously problematic. Sometimes dropping the device can cause issues that prevent the Nexus 7 from charging altogether. A hard reset may not work. Fortunately, it is quite easy to fix.
However this will void your warranty. Pop the back of the Nexus 7 case open by pushing a credit card or guitar pick or small screwdriver under the edge of the black plastic case. Start from the top and gently pry the case open, working your way around. The back of the case will come off quite easily. The battery occupies the bulk of the left-hand side of the unit, and a group of six wires come out of the top of the battery to a connector on the circuit board. Gently push the plug out of the socket connector, wait 15 seconds, and push it back in. Snap the case back on and plug in your charger. You should now see the battery charge image appear.