Windows is great for all these programs you can find, free or not, all these options, customizations and hacks, even the great variety of hardware, however, it is not easy or even possible for the operating system to be perfectly compatible with all the programs you may use.

The taskbar may disappear because some drivers need update, or a program, even antivirus, causes conflicts, etc. However, you can usually bring it up immediately and easily without logging-off.

Open the Task Manager (Ctrl+Shift+Esc) and go to File > Run New Task. In the dialogue that opens write explorer.exe and press the OK button. The taskbar appears, the Windows key and Start Screen are again available, etc.

Enjoy!