What version of Windows are we dealing with here??
I'll walk you through some steps on a Win7 Machine you may want to try...
If you go to the start button, right click on computer and Hit "Manage" a window for "Computer Management" should open.
From there, Open Disk management, and usually wait a few for the list to populate. if it takes longer than a few moments, either your computer is hella slow or you have a bad storage drive plugged in.
If you have been waiting and removing the USB Drive instantly populates the window, you may want to try the drive in another port or count it as gone completely.
Should the list populate, you should see a list of Discs, and a list of drives. Try to find the removable disk for your USB Flash drive! (you may find it useful to unplug any others to keep them safe during these procedures and to ensure you are picking the right disk)
Try right clicking on it and Ejecting it, subsequently re-inserting it into the computer.
If that does not work, and there is a partition with no drive letter, right click on the drive and attempt to assign a drive letter to it.
If that does not work, you can attempt to format and erase all of the information from the drive as a last attempt effort to salvage usability of the drive.
Outside of this are serious issues for the drive and it is suggested to instead replace it!