If you reinstalled XP, then it overwrote your GRUB boot partition. That would be why your Ubuntu doesn't come up anymore.
If the partition disappeared after you installed Ubuntu, then it's probably because XP doesn't know how to read the ext filesystem that is on that partition, so it assumes that part of the disk is just garbage data and ignores it. Your hard drive space is still there, just unreadable by windows.
I try to keep my data on a FAT32 or NTFS partition, since both windows and linux can read them.
You can follow directions here to reinstall GRUB (this will wipe out your windows XP boot sector, but GRUB should detect windows and add it to the list of operating systems it shows when your computer starts up):