Venue 8 7840 Won't Start

My Dell Venue 8 7840 is used daily since purchased in July, 2015.   Warranty expired in July, 2016.

This morning - after charging overnight in the usual way - the tablet would not start.   No lights, no activity - totally dead.   I know the battery is fully charged as it was almost full last time I used it.

Now it looks dead as it doesn't matter how long I hold the "on button".

Any suggestions?  Is there a master reset/reboot button somewhere?

Thanks for your help,
Herb...