The message means that the computer cannot find a device that is bootable. This can mean:
1. your disk died. Hope you have backups.
2. something (malware? buggy software?) wiped your master boot record. You can rebuild the MBR without losing data.
3. something wiped your disk. Time to reinstall. ditto backups.
4. you have a CD in the drive that appears bootable, but is not and the CD/DVD drive is higher in the boot order than the hard disk
5. you have a USB device that appears bootable, same as above
6. Something messed up the boot order in your BIOS and your disk is no longer in the boot sequence.
Post a new thread (not marked solved) if you need help.