How to recover flash drives

Hello,
I am a wild life photographer and having some of my photos in my memory card. When i came back from my photo shoot i found that i am not able to access my photos so can anybody tell me what happened and how come i can get my photos back. As one of my friend suggested me to do the recovery but to recover flash drives is not that easy.

I have tries software but it was trail version as software is good but was only the trial version..
http://www.recoverflash.com/drives.html

Regards
Eden