My tablet screen is broken and it won't turn on.

Well you need to know if the screen is broken or the device itself is broken. First check and see if the device still turns on. If it makes a sound or vibrates. then your device is still alive.

IF you are positive that the device is ON and the screen dosent show anything then you would probably have to change the LCD.

If you are doing the repair by yourself then try disconnecting the LCD and Reconnected it and see if it works. If not then you need to replace the LCD/Screen