Can you boot off a Linux CD/DVD and look at all partitions on your hard disk?
Sounds like you have a Ubuntu distribution, you can create a CD, DVD, or thumb stick with their ISO file and when you boot off that you get the option to run Linux from that media versus install it. Run from the media and it should attempt to mount your hard drive partitions. Either case, after all booted, get a terminal and log yourself in as root, there should be no password in this case. Then type
This will view the partitions on your SDA drive. Once you do that, I'd recommend you post it to the thread for further review.
There are file system tools which can be used to check your file system and repair it, but they depend on the type of file system which you have on your Linux partition.