Had the same problem a few months ago using a Lexar 16GB 1000x.
I lost a whole photoshoot and hundreds of photos without a chance to recover any
I tried at least 10 different data recovery applications, Even Lexar's own and nothing.
Called Then, they sent me a replacement card.
I had another 1000 X card with me since they were on sale at B&H. I started using the second card and five days later it died on me just like the first one did.
I called Lexar again to complain and they offered to send me a second replacement card. I actually forgot to send it back and I have been using two cards on my Canon 1DX, the lexar and one SanDisk simultaneously. That way if One cards craps out, I will have a backup.
It is unheard of for an expensive CompactFlash to fail twice in a couple of months. I am guessing it might have been a bad batch. I got mine around July or August of last year. I actually haven't had any problems with Lexar since the replacement card was sent to me. And I have used it every day for the past eight months.
I average 300 photos a day, so, give or take I must have shot over 60000 images on The memory card and it hasn't
Cross my Fingers it stays like this