If you've accidentally removed your Raspberry Pi OS taskbar and icons, there are a couple of ways you can get these back. The following is valid for all models of Raspberry Pi, including the Pi 400:
Terminal commands - open a new terminal window (via SSH or similar if you have no terminal icon) and enter the following commands:
rm -r /home/pi/.config/lxpanel
lxpanelctl restart
Reinstall Raspberry Pi OS - You always have the option of reinstalled Raspberry Pi OS on your micro-SD card if you have a card reader. Just download the Raspberry Pi imager tool and follow the instructions: https://www.raspberrypi.org/software/
Note: this will erase any files or settings