Thanks for this. I accidently hid the (1) critical sensor I wanted to see Mem junction temp on my GPU.I know this is well over three years old now, but I wanted to leave a helpful reply for anyone else that happens to Google this, since I accidentally hid a value that I meant to display and then couldn't figure out how to fix it for a bit.
If you want to restore the hidden values on the sensors screen, just go to the sensor settings page, then the layout tab. The hidden values will be shown at the bottom of that page, and you can simply click them, then check the box to show them in the list. You can also use the "restore original order" button to reset everything, but that will also undo any customizations you may have made to the display order of the sensor values.