Two things to try
1. On many desktops the plastic front has metal "bosses" which balloon and are split, so a well aimed screwdriver can lever the case front off when about 6 such places need easing. If it is that type then you should be able to see most of them from inside and at the corners.
2. On one case I had to fiddle with recently, there was no need to remove the front because the Floppy and HD were mounted on a steel twin shelf. I could remove the lot if slid sideways to disengage the tabs at the top and bottom of the 'cage'- there may have been one or perhaps two self tappers holding the gizmo