This message means the hard disk is not bootable. Either something simple happened (e.g. MBR got corrupted) or the drive failed.
Boot computer and get to BIOS. See if the BIOS recognizes the disk. If so then download a bootable diagnotic disk (google ultimate boot disk). Run a disk diagnostic and see if the disk is OK.
If the disk is OK, then try to boot off the recovery partition on the drive.
If the disk needs to be replaced (simple task, instructions in the manual online, disks cost $100 for 1TB or 2TB drive) or if the recovery partition is wiped then you need to order recovery media from compaq/hp. Typically this costs $10.