It has been brought to my attention (by my daughter who thought she had broken something) that some of the 7" Tablets using Android 4.X allow the user to turn the brightness down to a point where the LCD tuns off and will not allow you to see the screen to turn it back up....
Step 1: Unlock
Here are the measurement from my tablet. They should give a fairly good idea about where to find things on you screen. Remember, everything is right where you left it, you just can't see it. When you succeed in doing the right pattern in the right place, your tablet "should" make a happy unlock noise.
Step 2: Open the Menu.
After the happy unlock noise, swipe down from the upper right hand corner.
Step 3: Turn Up the Brightness
Using the supplied measurements, tap the area where the "BRIGHTNESS" button should be. The slider bar will appear for 5 seconds near the center of the screen. You shouldn't need to slide, just tap near the right side to get the screen bright enough to see. From there you can refine it!