Flash drives are getting to be more and more common, but unfortunately, they're still very susceptible to failure, through abuse or electronic wear and tear. Here's what to do if you've got a data loss issue on a flash device.
Prevention is the best way to avoid data loss, so a few quick words on that before we continue. Backing up information on a flash drive is very easy; simply plug it into a computer and copy the data to a folder. If your flash drive contains sensitive information, try using the free software Truecrypt to encrypt into.
It's important that all valuable data be stored in at least two physical locations, preferably far away from each other, for backup to be effective. If you're able to backup your flash drive on a consistent basis, you won't need to explore data recovery methods.
Now, if you didn't backup properly, that might be OK. On to recovery...
There are two routes to go depending on what happened to your flash drive. If it's intermittently reading or you deleted the data by accident, you'll want to try software; if it's physically damaged, you'll need a professional data recovery company.
The software I recommend the most is for pictures is Klix, a flash recovery tool provided by ProSoft Engineering. Deleted files that haven't been overwritten will be easily pulled back, there's a fantastic user interface and very little risk of messing something up if you do it wrong (the program's built to avoid nasty user mistakes--so, yeah, it's idiot proof). It's also one of the few data recovery programs developed specifically for use with flash media and pictures in particular. Other data recovery programs will work OK for logical errors, but unless there's a program that you've already purchased that you want to try on your card, go with Klix.
Prosoft's own Data Rescue II is my choice if you want the most in-depth program possible, or GetDataBack, though that's more of a professional tool, so read the documentation carefully. Try to use programs with trial versions that let you see what they can recover before you buy.