This message indicates that someone changed the default login shell for root to a program that is missing from the system. For example, the final colon-separated field in /etc/passwd could have been changed from /sbin/sh to /usr/bin/bash, which does not exist in that location. Possibly an extra space was appended at the end of the line. The outcome is that you cannot login as root or switch user to root, and, thus, cannot directly fix this problem.
The only solution is to reboot the system from another source, then edit the password file to correct this problem. Invoke sync(1M) several times, then halt the machine by typing Stop-A or by pressing the reset button. Reboot as single-user from CD-ROM, the net, or diskette, such as by typing boot cdrom -s at the prompt.
After the system starts and gives you a # prompt, mount the device corresponding to the original root partition somewhere, such as with a mount(1M) command similar to the one that follows. Then run an editor on the newly mounted system password file (use ed(1) if terminal support is lacking):
# mount /dev/dsk/c0t3d0s0 /mnt # ed /mnt/etc/passwd
To keep the No shell problem from happening, habitually use admintool or /usr/ucb/vipw to edit the password file. These tools make it difficult to change password entries in ways that make the system unusable.