First, do you know off hand what filesystem is used? If it is Solaris's zfs, then you need a live distro or rescue disk for Solaris.
When you run off a live distro or rescue disk, the root filesystem exists in memory. Look at the output of the "mount" command by itself to see if the hard disk partitons are mounted. If not, you will want to mount the root partition somewhere such as mount. There are a lot of ifs here, such as whether the kernel your live distro or rescue disk uses is able to mount the partition rw.
Solaris will use different device names for partitions. There are in /dev/ and probably end in p1 for the first partition, p2 for the second, etc. If your disk has the fdisk program (I don't know if solaris has its own version of this program), running "fdisk -l" will list the partitions.
You need to use the mount command to mount the partition that has the /etc directory. It should be on the root partition. I can't see anyone wanting to mount it anywhere else.
Look at the /etc/passwd file. If there is a line like: "root:x:0:0:root:/root:/bin/bash" with the second field of "x", then the system uses shadow passwords. The password hash is in /etc/shadow. Otherwise, you may see a lot of random characters between the first and second colons.
Look at the /etc/shadow file at the entry starting with "root":
Delete the characters between the two colons to zero the root password. This means you can reboot and login as root with no password. Then run "passwd" as root to change the root password.
I have a system that doesn't allow a null password for root. So I couldn't log in after zeroing out the password hash in /etc/shadow. So I copied the hash from my regular users entry and copied it for root's. That allowed me to log in as root using my regular user's password and then I ran "passwd" to change it.