This just happened to me and it took me a lot of research to figure out that was the problem. I even called Sony Vaio support (third party provider) who wanted to charge me $99 to help trouble shoot the issue but luckly I found this solution instead. Turns out it was a recent Windows update (Aug 2016) that secretly turned it off for me.
If you're not used to Windows 8, getting to the list of devices can be difficult so I recommend just typing "Device Manager" in the Windows search box, expand the list of Human Interface Devices and find the USB device that has been disabled, then Enable it. The touch screen should become active immediately.