You may find some help here under the section titled "Lost Partition" https://help.ubuntu.com/community/DataRecovery
Lost Partition
If you made a mistake while partitioning and the partition no longer
appears in the partition table, so long as you have not written data
in that space, all your data is still there.
GNU Parted
Run Parted from the command line to recover your partition.
When changing the partition table on your hard drive, you must ensure
that no partition on the disk is mounted. This includes swap space.
The easiest way to accomplish this is to run the live cd. Parted is
installed on the base Ubuntu system. Once at the desktop, open a
terminal and run_:
sudo swapoff -a
Next run parted and tell it to use the device in question. For
example, if your /dev/sda drive is the drive from which you want to
recover, run:
sudo parted /dev/sda
Then, use the rescue option:
rescue START END
where Start is the area of the disk where you believe the partition
began and END is its end. If parted finds a potential partition, it
will ask you if you want to add it to the partition table.
Testdisk
Alternatively, the testdisk application may recover your partition.
Use any method to install the testdisk package.
Run testdisk and it will scan your computer for media and offer you a
menu-driven way to recover your partition.
sudo testdisk
Gpart
Another program that can scan drives and re-create a partition table
based on "guesses" is Gpart. Use any method to install the package
gpart.
To scan the first hard disk using default settings type
sudo gpart /dev/sda
or
sudo gpart /dev/hda
depending on your Ubuntu version.
You can restore the "guessed" partition table, only after checking it
very carefully (you're strongly advised to write to another device
instead), using
sudo gpart -W /dev/sda /dev/sda
