You can't recover it, but you may be able to change it. Now I can't give specific advice, because I'm not familiar with Debian on the 68k. But you should be able to tell the bootloader which runlevel you want to startup the system in. Try runlevel 1, single user mode. Hopefully you'll get access to the root account without being asked for a password (common on older versions of Linux, but it's less common today). Then use passwd to change the root password.
The other option is to snag the passwd file from the Linux partition, remove the password field from root (with BBEdit, for example, since it handles Unix line-endings), then replace the old passwd file with the edited one.
I wish that I could be more specific, but I've only ever used NetBSD on 68k Macs.