If you ran
usermod -s /usr/ root as root then you have set root's shell to be the usr directory, which is invalid and will not work.
/usr/ is a directory, not an executable. If you actually ran
usermod -s .usr/ root then you probably set root's shell to a nonexistent file, which is invalid and will not work. You should have said
usermod -s /usr/bin/ksh root or some other actual executable.
If you can log in as a regular user you can run
su -s /bin/sh root -
Which will get you in as root with a working shell. From there you can fix your assigned shell with usermod.
And remember to take care in the future when working as root. Most commands assume you know what you're doing and that you won't make any mistakes.