doffen, I’m not a specialist on it…
I guess it could be two things: the MBR (master boot record) messed or the Partition table messed.
If you boot with a third party CD of Partition Wizard, you should “see” the partition (and not a free space). If you can’t see the partition, try partition recovery option.
If you see the partition there, maybe, follow this thread (read carefully): https://forums.comodo.com/help-ctm/how-to-restore-mbr-if-you-cant-boot-withwithout-ctm-t58885.0.html