This happens to me when a programs want's to give me a notification and windows is set to not show the notification.
The program in question just can't deal with it and keeps trying and the silly taskbar won't go away.
The only solution I have found short of reloggin/rebooting is to open the task manager (Ctrl+Shift+Esc) then find and kill explorer.exe.
Then in the run menu of task manager type "explorer".
Your desktop comes back, all taskbar and notification area icons accounted for (unlike in previous version of windows) and more importantly, your taskbar will now autohide.
No running programs should be harmed.