Could be a failure of the drive controller on the motherboard.  Check though
that it isn't simply a cable problem and that you have it the right way round
and firmly seated at both ends.

Check your BIOS settings:

should be 3.5" on A: and *not* "Type 3"

Then check your cable

- seated properly
- connected at the A: part to the FD
- red stripe matches Pin 1 at both ends
- give a shot with another cable

Rick