Under the General Tab, make sure there is a check mark next to the Hard Disk.

If there wasn't one, put it in and the HD icon should appear on the desktop.

If there was one, and the HD icon is still not on the desktop, go to Your account, Library, Preferences and drag com.apple.finder.plist onto the desktop.

Restart the mac.

Did you get the HD icon back?

What you dragged out of the sidebar was an Alias of the HD. Dragging it, and any of the other icons you see there, just takes it out of the sidebar. Rather like dragging something off the dock. Just goes Poof.