What is disappeared is the Applications Menu with the logo of Xubuntu.
How to bring it back? I can use the terminal but I don't know what to do exactly.
I have just tried to reboot the system from the terminal but the bar doesn't return when the laptop is on again. Every suggestion is precious.
You can add any items you like to the panel and choose their position.
The item is called Whisker.
Use panel properties for that.
XFCE is very customizable ;-)