hard drive - Whole working Windows 7 partition (on SSD) suddenly became "Unallocated Disk Space"

I have an HP 2540p with Windows 7 installed on a Plextor M3 SSD that sits in the upgrade drive bay (that used to house a DVD drive). The system has been working fine for months until this morning when I turned it on (after a successful hibernation the night earlier), I got a

Non-system disk. Press any key...

on a black screen after the BIOS POST screen.

Here is the list of what I tried

1. Using HP "HDD Self Test" available in the BIOS: The test finishes and does not report any issue (note that HP documentation on this mentions that if it's a "soft problem" on the hard drive, it will be fixed automatically without notification.)




2. Using a Windows 7 repair disk: The repair environment does not see the Windows partition (the SSD).

   
   

   
   

   DISKPART > LIST DISK

   
   

   
   

   does see the SSD, but showing "0 B" free space.

   
   

   
   

   DISKPART > LIST VOLUME

   
   

   
   

   does not show this SSD.




3. Putting this SSD into a USB case and plugging it into another Windows 8 computer: This is where I found that Disk Management says the drive is "Not Initialized" and thus the space of the whole drive is "Unallocated".

There are many important work files on the drive and only part of them (that are not too large) have been back up. So it is better if I don't lose the data.

If anyone knows what caused this problem and how it can be solved, I'll be greatly appreciated. If this has been discussed before elsewhere, please kindly point me to the discussion or suggest a keyword for google search.

Thank you very much and have a good one!

Pawin