The first thing you need to do is install gparted with the live CD. That takes a few steps, but the best tutorial I found is right here.
Once you finish with that, we want to check our current partition table. Open up terminal and use the command
cat/proc/partitionsYou can view the details about the disk by using this command:
sudo fdisk -l /dev/sdaNow we need gparted to guess what it thinks the partition table used to be.
sudo gpart /dev/sdaAfter a while, it will output it's guess. Take a look at it. If it looks reasonable, then go ahead and accept it by using this command.
sudo gpart -W /dev/sda /dev/sdaFor most cases, your partition table should be repaired.