I have a one plus 3. It runs Android Oxyget.
Long tap on screen, then choose "Add widget", then find google search widget and place it on screen where you want.
As Alex.S said above, you can re-add the widget. However, if you removed/disabled the app, here are the steps to get it back:
- Check if the app
- If it is, touch it, and hit the Enable button.
- If it is not, go to the Google Play Store, and reinstall Google by searching for it.