If your taskbar disappeared and doesn't even come back after rebooting the system, start the Windows Explorer by hand.

How do you start a program when there is no start button? You can press Ctrl + Alt + Del, then open the Task Manager. Click on the Applications tab, then on the button to start a new task. In the dialog box that appears, enter: explorer

Hit return and check whether your missing taskbar reappears. If it does not, you have a problem other than the one described here and this article cannot help you. If it does, please read on.

The cause can be that a particular registry value has been changed by some software. You can fix this manually as follows.

Open the registry editor regedit. If you cannot find it in your menu, click on Start, Run..., and enter: regedit

In the registry editor navigate to the following key and value:

HKEY_LOCAL_MACHINE
 \SOFTWARE
  \Microsoft
   \Windows NT
    \CurrentVersion
     \Winlogon
Value: Shell REG_SZ Explorer.exe

If your Shell value is Explorer.exe, then this article cannot help you and the cause is something else. But if it is different, for example, if something else is there besides Explorer.exe, double-click on "Shell" and change its content to only: Explorer.exe

Reboot for a final test. You can do that by pressing Ctrl + Alt + Del again and selecting to shut down the system and restart it.