go into start/settings/controlpanel/system and find on device manager. Make sure that the floppy drive doesn't have an X on the icon. If so or if not there then click refresh and use the add new hardware to re-add it. Or delete it if it is there andreboot to auto-detect it.
Also are you sure the that the floppy disk you tried to use isn't bad?
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Floppy drive not working