While in game I received a Blue Screen, which is rare on this system.

After rebooting Game, I noticed I lost data.

Multiplayer prompted for my Key. All five (5) of my custom classes were gone (Weapons, attachments, unlocks, etc).

Can I reconnect to data, file, on /c drive? How do I regain level 28 work?

Ok! Found it myself. Data or pointers may be lost with a Blue Screen crash. I created a second profile and played on that profile. The pointer to that profile was lost. Once I recreated the Profile "tag", the game pulled up my level 28 data. Whew!

Were do I find profile data, so I can back that up, please?

xsSkorpio

I had this exact same problem. After a BSOD, the 3 .xml files that were associated with my profile (in the Saved Games folder) were blank, but still registered as having a file size. I fixed this by copying the 3 .xml files from the "default" profile and overwriting them with the corrupted ones in my profile. I am 99.9% sure there is no way to recover your progress, but your saved games for the campaign will remain intact. I fixed my BSOD's by removing my CPU overclock.