If your nexus 7 goes dead due to battery being completely drained out, there is a good chance it won't start / charge even if you keep it on wall charger for 24 Hours. I have faced this.
In such cases, keep the nexus 7 connected to laptop / computer for an hour or so and then start the tablet, it would come back to life and then use wall charger to properly charge it.
May not be applicable for your scenario but mentioned just in case if someone faces issue in bring nexus 7 back to life after being shut off due to battery empty.