First of all it would be nice to know wherher it is the flash drive itself, your computer hardware or the software that is acting up. What did you do before this happened?
Take your flash drive to another computer and see if it works there. If you don't have another machine around, try rebooting your computer. Plug the lash drive in a different USB port - if you tried the one in the front then go and plug it in the back now. Look in Windows Device Manager and see what it says about your USB devices.