The taskbar at the top of the desktop in ubuntu ,which I have lost , cant restore again.I'm unable to access many programs,please help me.....

You still have the bottom "Task Bar" actually called Panel at the bottom. You can just right click it and create a new Panel and put it up top. You can add your programs menu back to it. Just right click your new panel and select Add to Panel. There are many things you can add to it. You want to add either Main Menu or Menu Bar first to get back what you are wanting.....

I'm running Ubuntu 9.10 (64 bits). A couple of days ago, the "Applications" option on the Menu Bar suddenly stopped working. "Places" and "System" still work fine, though.
When I click on "Applications", just a small white rectangle opens under de Ubuntu's logo, but nothing else happens! The "System/Preferences/Main Menu" option doesn't do anything, nor does de right mouse/"Edit Menus" option, either.
I've tried deleting the Menu Bar and adding it again, as well as the command
sudo dpkg-reconfigure gnome-panel
to no avail.

Can somebody help me, please?