Pretty common problem. You likely killed your window list on the gnome panel. Try right-clicking a gnome panel and looking for an option to add a new panel, right-click the newly added panel and select add to panel, from this choose Window List.
Alternatively, if you already have the panel that the windows should be minimizing to (bottom one, if my memory serves me), right-click it and select the add to panel option and choose window list. I'm sorry I can't give more specific instructions but I'm stuck on my Windows 7 work PC