Go to my computer, right-click on the flash drive and select Properties, go to tools and run disk check on the flash drive.
You will most likely have to reformat it.
If you know how, you can make a liveCD for linux and boot into said liveCD and check to see if the Fash-drive is recognized inside of linux. If it isn't reformat it. If it still can't be accessed. It might not work anymore.
If you need to get files from it, you might be able to with http://www.piriform.com/recuva
Do you remember what filesystem you had on the flash drive?Ex: NTFS, FAT, FAT32