KDE 4.0 is still buggy. Plasma crashes a lot and quite surprisingly the applications panel disappears. No worries, let me show you how to bring the panel back.
1. First, kill/close the plasma process. One easy way is to cntrl + escape, search for plasma and kill it.
2. Your desktop disappears. Press Alt + F2, to bring in the kicker and launch the terminal.
3. Delete the plasma configuration file and relaunch plasma
2. Voila!! Panel is back.
Sad part is you need to reconfigure your desktop, icons and widgets.
PS: Do read the Emergency release notes FAQ and the KDE 4 visual guide