Hi Ellen,
Unfortunately, there are quite a few reasons the touch screen isn't working.
It could be there's a hardware problem with the digitizer, which would mean replacing the screen, as the LCD and digitizer are fused together on your phone.
It could be a software problem as well. There are several things you can try, including an operating system update or a full restore. First thing to try, though, is a hard reset of the phone. Here's a guide to show you how (it's for an iPhone X, but is equally applicable to the XS).
[guide|99182|How to Force Restart iPhone X - iFixit Repair Guide|new_window=true]
Give that a try and let us know what happens.