The update should not cause the unit to not turn on. Most likely the battery has depleted completely. If that is the case try the following steps and see if that works.
1.Connect the charger to your Nexus 7, then plug the charger in to an outlet.
2.Press and hold Power until the device powers on, then immediately press and hold Volume Down (while still pressing Power). You will see the word Start with an arrow drawn around it.
3.Press Volume down until you find Power off, then press Power to select that option.
4.When your tablet is off, disconnect the charger, then reconnect it.
You should see a battery on your screen that indicates your tablet is charging.