the answer to your question "whether is it possible the lost data from a USB Flash memory" is , "it depends." If your data was properly moved to the flash drive and is not overwritten by some other data then Yes it is possible to recover it. One very common cause of data disappearance from removeable media is due to presence of a particular malware. These malwares hide the actual data from the media to pretend as if it is gone .Try the following steps:
1. Update your antivirus software.
2. scan your USB flash drive completely.
3. remove all malware or threats indicated by AV software.
4. go to my computer and from the top menu click on view-> Folder options-> check show hidden files and folders option from the list. (I assume you are using windows xp)
5. double click on my computer and note down the drive letter assigned to your USB flash drive, lets say it is "H".
6. type "h:" in the address bar of my computer. (it is a precautionary step to avoid reactivate the virus just by clicking the USB drive in my computer)
7. you should see all your data present there. just copy what you needed and you know you put there at some other location.
8. After taking a backup of what you needed from flash drive, format the drive.
Let me know if all this work for you....
otherwise I will tell you some other remedies.