Question: After I installed GNOME Shell, `CTRL` + `ALT` + `T` combination stopped working.

How could I fix it?

Answer: 
Go to **Settings** -> **Devices** -> **Keyboard**, then add `Ctrl` + `Alt` + `T` if it does not exist.

[![screenshot](https://i.stack.imgur.com/KcM9s.png)](https://i.stack.imgur.com/KcM9s.png)
Upgraded to 18.04 from 16.04 ; kept my preference for the Unity desktop. But couldn't open the terminal with a key combo despite checking the shortcut in the checked answer.

* Install `compizconfig-settings-manager`
* Run `ccsm`
* Turn off the "Commands" plugin

This resolved the lack of `ctrl-alt-T`, `Vol +/-`, all the other extended and media keys on my keyboard.