If your device will not power on or off, follow one of the links below for manufacturers' troubleshooting guides:
If your device was made by a different manufacturer, you should be able to find relevant guides online.
Samsung - Soft Reset
Samsung A7 Lite / SM-T220 - Wont power on
1) Charge device - Leave on charge for a long time, ideally with a powerful charger
2) Plug into laptop/computer that is powered on and connected to power and leave to charge for 30 minutes
3) If it doesn't power on normally, keep connected to the laptop/computer press and hold power button for a few seconds
4) If it doesn't power on keep connected to the laptop
A) press and hold the Volume Up and Power buttons simultaneously
B) When you see the Samsung logo, release the Power button.
C) Wait for a menu will appear, release the volume button.
D) Select reboot using volume buttons to move up and down the menu, and the power to select