LXPanel is the standard panel of LXDE. I don't know what causes it, but I haven't seen it happen on Buster (yet), but if it happens for you, try this: lxpanelctl restart. This is a message for the developers of PIXEL desktop. I found the process running to be: lxpanel --profile LXDE-pi process. Part six(!) in my Linux Desktop series, I'm looking at LXDE on Debian GNU/Linux and Raspbian.