I had this with 2 brands of USB flash sticks. Caused by trying to run Toshiba Recovery Media Creator on Windows 7that fails and "wrecks" a working USB stick with exactly the symptoms you describe.
I fixed it like this (Windows 7)
Log in as Administrator.
Insert the USB Stick
Go to Control Panel
Open Admininistrative Tools then Computer Management
Select Storage and Disk Management
Select the disk that is your USB - it will be shown as unallocated and have a black band on the top unlike the blue bands on your hard drive. Take care NOT to select your Hard drive
Right click on the panel of the disk that says unallocated
Select the top item New Simple Volume...
Take all defaults and allocate a new drive letter.
Finish and the drive will be working again.