How do I get my taskbar back to default?

To do it, right-click on the taskbar and select Task Manager from the options. It will open the Task Manager. In the Processes tab select Windows Explorer and click on the Restart button at the bottom of the Task Manager window. Windows Explorer along with the taskbar will restart.

How do I fix the messed up taskbar in Windows 10?

Quick fixing

A possible workaround is described by ‘old_old_geek’ on Reddit: “After booting has finished, right click on the taskbar and select Taskbar settings. Toggle the setting for ‘Use small taskbar buttons’. After the taskbar is redrawn, toggle the setting again. This works for me.