Panels are missing; dependency issue

[mehta]

Xubuntu 9.04 loaded ok but after the first update, I think, the top and bottom panels have disappeared.
I am able to run the panel using xfce4-panel and through that learnt that there is a problem with my installation. When I close the terminal window the panels disappear again.
On running apt-get I got the following output:
mehta@mehta-laptop:~$ apt-get check
E: Could not open lock file /var/lib/dpkg/lock - open (13 Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
What should I do next? Please help.

[john.nicholls]

After you get the panel back by running xfce4-panel you need to save the session so it'll come up again on reboot. You can do this by right-clicking a blank part of the panel and selecting Restart. Alternatively, go to Settings | Session and Startup and tick Automatically save session on logout.