What could be the reason for dock icons to have disappeared?
That is a strange behavior which happens to many users. One way to fix that is to open a terminal and use the killall-command.

Because the launchpad is connected with the dock, you can just use the following command to get your icons back:

killall Dock


That should fix your dock and launchpad-problem. But I don't know if it will help for the missing Desktop-icons.