" But when I reboot from win98 boot disk and use fdisk /mbr, it shows no disk."
What do you mean with that? If you're waiting for a confirmation or anything you won't get it. What happens is that the command starts fdisk and when that's done you'll get back the command prompt. THat's all there is to it.
I don't know if fdisk or cfdisk for linux has the same command.
but as far as i can see, none of them have.
Use fdisk from your bootdisk.