You might try UNchecking it - rebooting - REchecking it - rebooting - try to drag the Taskbar again.

Just another thing about dragging the Taskbar ... You will not see it in transition (moving) but once the cursor is on another edge of the screen it should reappear.

If not try System Restore. Are you having other problems?

Do you run an Anti Virus and scans? etc.