According to this
page it looks like it is part of the gnome-applets
To re-install (but I would try to re add it to the panel first):
sudo apt-get install gnome-applets
Then to add to the panel, right click on the place you want the Sticky notes applet to appear, select Add to panel..., Sticky notes.
It will appear as a yellow icon that looks like a block of post-it notes.