I know its very simple solution, but first time when I got this issue I was surprised and I got no solution.
Mistake : I was about to remove some extra panel from my task bar. But I deleted the panel.
Outcome : I lost my bottom taskbar, whatever window I open I can not see I always need to go Alt+Tab for that.
In top panel right click on your panel and left click on “New Panel” Make sure new panel is at bottom – if not click and drag it to the bottom.
Once your panel is at the bottom – right click on the panel and say “add to panel”. A menu will open and from the menu select “Window List”. Left click on the “add button”.
Now when you minimize the window it will appear at the bottom panel.
Hope this will help.