Most tablets and cell phones have what is called a digitizer which is a thin almost like glass reactive layer that covers the screen. This connected to the inner workings of the device with a band of wires. If yours is broken in the right/ wrong place then it cannot react when you touch the screen. If your actual screen were cracked you'd usually see what looks like paint or melted crayons running down the screen which is the liquid crystal that makes up the actual screen. The digitizer how ever is fairly easily replaced and can usually be bought for between 15.00 to 50.00 dollars US. You can also Google search your exact situation and find an ideo in diagnosing the problem and how to repair or change it.