Damaged Hard Disk or Disk Controller. Contact Technical Support. Hard Disk directory or FAT is scrambled. Copy your backup data back onto hard drive. Backing up the hard drive is extremely important.
All Hard Disks are capable of breaking down at any time.
System only boots from Floppy Disk. Hard Disk can be read and applications can be used, but booting from Hard Disk is impossible. A number of causes could be behind this. Back up data and applications files. Reformat the Hard Drive as described in the Hard Drive section of this manual.