Auto-hide Taskbar won't unhide

I do believe this is an OS bug which has made it into every Windows version since (at least) XP. It's also difficult to reliably reproduce but does affect numerous people according to Google, myself included, which is one of the reasons why I always end up disabling it shortly thereafter.

You could try one of the following, albeit neither is a clean nor permanent solution:

• Press the Windows key + D to minimize all open programs. Afterwards the taskbar may react normally again.
• Kill and restart Explorer via Task Manager. To restart Explorer from within Task Manager:
  File -> New Task -> explorer.exe -> OK