If your Nexus 7 tablet doesn’t start (the screen stays black), don’t panic.
This issue happens if the tablet didn’t had power for a day, or half a day. If the battery has stayed empty for that long.
What you can do is to leave the tablet to charge for around one hour, and then try to start it.
If it doesn’t start, it is possible that the charger isn’t connecting as it should.
You can clean the connector that goes into the tablet and also the USB connector that goes in the power charger.
You should use a contact cleaner spray. Just spray on the contacts and let them dry for a few hours.
If your tablet still doesn’t turn on, you can just contact the store from where you bought it. The Nexus 7 has around 3 years warranty. If you don’t have a warranty, just take it to a professional that fixes these kind of devices.