In most cases it means the partion has been whiped.
That would imply that something was done to wipe the partition. I would think it more likely that the drive is not being read properly, perhaps can´t be read. The machine is old, isn't it Dinga? So one must recognise the possibility that the drive is dead or dying.
If the machine is not booting from a bootable DVD, then it is not finding, or not looking for, the medium during boot. Are you sure that the DVD drive still works? When was it last used?
If you are sure the DVD drive works, and the medium in it is definately bootable and ok, and the drive is not starting at boot, then I think the problem must be in the BIOS. That is to say, the problem with the DVD medium, not the message that there is no operating system.
Resetting the BIOS as Cromag suggested may help. It can't really hurt, either. The machine is apparently pretty comprehensively broken, so resetting the BIOS is unlikely to make things worse.
Have you, in the past, sucessfully booted the machine from a USB medium? If so, do you have a bootable USB medium at hand? You could try that.
If you can get the machine to boot from a DVD or USB, the question is whether the hard drive is being fouind or not. If not, and if the machine is old as I assume, then I think it is highly possible that the drive is at least sick and maybe dying or dead already.
If you can determine that the machine is otherwise ok but not finding the drive, i.e. get it to boot from a bootable DVD or USB, you could try putting the drive in an external exclosure to see if another machine can mount and read it. If that works, the problem is in the hardware of the machine (probably, I think 😉 ).
You can then at least acces any files that are on there.
Or maybe it is really a corrupt partition. If you can get the machine booted from a bootable medium, or get the drive mounted from an external exclosure on another machine, you might be able to save something with a partitioning tool that is smart enough to block bad partitions. However, if the drive is starting to have bad blocks in a big way, I wouldn't be trusting it for important things.