It may happen when you disconnect the storage device (flash drive) without using "safely remove hardware" option, then, when you connect the drive the OS will say "You need to format the disk in drive X: before you can use it.".
Typically such a behavior means that you have a RAW file system. Obviously, initial filesystem on your drive was corrupted because the data stored in the buffer during disconnection of the drive was lost due to inappropriate disconnection.
You can quickly solve the problem by simply formatting the drive, but be aware that the data would be gone forever according to the format level you use. If the data is important you need to retrieve data first and only then format the drive. Data recovery in case of RAW filesystem is like unformat and easy to do - just download some data recovery software.
