Depending on the reason there are a couple of different solutions. If the screen is frozen, it will need to be rebooted. Either by holding the power button down for 10 seconds, or the power button and "volume down" button at the same time for 10 seconds. After it restarts, the touchscreen should be working. If it's still not working, the digitizer may need to be replaced. You can find the steps in more detail on the troubleshooting page
Try rebooting or if that doesn't work a reset Hold power button and volume down button together for 10 seconds. If these did not work there is a chance you damaged the digitizer if the tablet was droped or suffered a blow of some sort.
