How did openSUSE trash your Mandriva? Did SUSE partition the HDD where Mandriva was and install over it or is the problem with the bootloader?
If you can boot either, open a terminal and as "su" run the command "fdisk -l" to look at your partition table and post it. Otherwise, you can use a live-cd.
I always keep a copy of PartedMagic at hand for cases like this.
If the problem is booting, you might use the live-cd of the last linux installed to fix the bootloader and chainload the other linux.
You can also use a copy of the SuperGrubDisk to repair GRUB. There is useful information in the wiki.