1- Do you use any magnetic cover case ? This tablet has a magnetic sensor that will shut down the screen.
If you're using one, try to use the tablet without the magnetic cover.
2- When waking it up, does the button menu and back light up ? If they do, your tablet is on but no display.
There is no specific answer as there are many sources to cause this problem.
You can go directly to Factory Mode to test the Sleep fuction
1- Use calculator to enter this code to go to Factory Mode
(+30012012732+
2- Then enter *#0*# to get to test mode

Touch the Sleep button, the tablet should go to standby.
Press Home or Power button, it should wake up.
Exit the test and factory mode by using the Back button
Factory Mode is the best way to test hardware functions.