Steam has lost my downloaded data.. I need help please..

[Lion]

Firstly I'd just like to say hi, and that this is my first 'discussion' here on steam.
I am relatively new to all of this, please be patient with me.

Yesterday I started downloading Planetside 2 and got just over 6Gb downloaded.
I then quit steam, and shut down my computer, nothing out of the ordinary.. I come back today to resume the download, but it is starting again at 1Gb into the download, the other 5Gb are completely missing..

I haven't had this issue before, and im sure its nothing that i have done (everything shut down and booted without a hitch just as I have done before with other download proccesses)
What is the problem here, am I going to download another 6Gb, only for it to disapear on me again? Is there anyway to retrieve the data I have already downloaded (without having to download it again)

Luckily I'm on an unCapped line. I couldnt imagine how much more frustrated and disapointed with Steam I would be if this was eating the last of my Cap for no apparent reason.
Please be so kind as to fix your ♥♥♥♥ Steam, or let me know what i can do differently to avoid such loss.

Looking forward to a response and hopefully some guidence.
Thank you.

[Ûvatha]

You can try to Verify the integrity of the game cache

Verify integrity of game cache

    Load Steam
    From the Library section, right-click on the game and select Properties from the menu.
    Select the Local files tab and click the Verify integrity of game cache... button.
    Steam will verify the game's files - this process may take several minutes.
    Once the process is completed, the Check Window will automatically exit.

If that doesn't help you have to download everything again.

[Lion]

Thank you for your help.
I did try what you said there, but alas it did not work..
I guess I'll just have to start again..