Hours ago I have lost my Ubuntu 10.04 machine's "Fast User Switch Applet". It was missing from Top Bar. Hours of searching, at last I have discover it. It is "Indicator Applet Session". I discover it in 'Add to Panel' dialogue box. Now I am able to do normal jobs as usual way. I can do:
I also get back my "Me Menu". I am available online again. Now I can chat, broadcast or use/ share Ubuntu One account. I also able to show status as 'Available', 'Away', 'Busy', 'Invisible', 'Offline'. Everything is normal now.