Press the home button on the tablet (using a paper clip if using a Feedbackly terminal stand) to confirm that the screen simply is not in sleep mode. If the screen continues to be completely dark and does not respond to touch or if the terminal shows a "empty battery" logo try the following:
Plug the power cord into another power outlet and leave to charge for 3 hours. The tablet charges slowly and will not allow itself to be activated before it has a certain amount of charge.
If the terminal still does not turn on then the charger might be faulty. If using a tablet then try charging it with different cables and chargers to see if either the charger or cable is faulty. If using a Feedbackly terminal then contact firstname.lastname@example.org for further help.