I suppose you can allways boot from a floppy with an msdos system on it and you will allways get to a: , or there must be something wrong with the bios , or the floppy must be defective , or badly connected .It can be a lot of things ,
I remember having a computer with only a floppy and no harddisk working perfectly (The old Atari st).
If you cannot get on your harddisk after getting to a: , it can allso be a defective mbr or another problem .