I installed Ubuntu 12.04 along with Windows XP. But now on the boot menu Ubuntu doesn't show up. I checked the drive in wich i installed Ubuntu, and that drive is also empty.. What do i do now?
Windows cannot see the contents of your Ubuntu partition
Ubuntu uses the Ext filesystem by default. Windows doesn't know what this filesystem is, so unless you specifically told Ubuntu to format its partition using NTFS (the Windows filesystem), Windows will see it as empty.
To see if the partition is actually empty, boot into your live CD again and see if it can see the contents of your Ubuntu partition. Chances are, it will. If not, then something did, in fact, go wrong with your installation and you should try again.
Grub is probably not loading right for some reason
Sometimes, Grub, the little program that allows you to choose which operating system, doesn't get installed quite right. (Sometimes, it conflicts with the Windows loader, which serves the same purpose.) There are a couple of things you can do to fix it.
It could be some other edge-case problem
If you do one of the two options in the "Grub is not loading right" section and you're still not getting anywhere, it might be something like this problem, where OS Prober got disabled somehow.
Another possibility is that Ubuntu somehow didn't get added to Grub's menu list. This post explains how you can temporarily edit the command that Grub runs, which may allow you to boot into your Ubuntu install. From there, you can install a Grub customizer to edit your Grub list. This is a rather advanced option and so should probably not be used unless you've tried the above and are comfortable with the command line.
If none of these things work, then you'll probably need to post more information to this question so that we can better determine the underlying cause of the problem you're having.