haha. i've had that before....
I assume your using gnome
I havn't used gnome in a long time, but from what i can remember, right click on the taskbar, i think its "add to panel" or something like that.
then theres a long list of stuff you can add to it.... theres something in there that holds minimised windows to the taskbar, i can't remember what it is called.... but you must of accidently removed it previously.... i think its called "window selector" or something like that. try that, if its not that, something that sounds similiar... cause im not sure what it was exactly. But it will fix your problem when you find the right one. lol.