You are either booting off the wrong device (possible but not likely) or your boot device has died a horrible death.
Boot cdrom -s from the ok prompt (after inserting a Solaris bootable cd into the cdrom drive. You will get a # prompt once it is booted.
You can then check out the drive to insure it is stable (run fsck against it, check that the system 'sees' it, insure the bootblock is installed). If you aren't suppose to be booting c0t0d0s0 as the boot device, then possibly it was changed in the EEPROM to the wrong disk and you only need to change it to the correct one - sometimes it's c0t3d0s0. If you don't know, then you need to check out your /etc/vfstab from booting cdrom single user and mounting your different drives till you find it.
If you have a alternate boot disk or a backup boot disk I would suggest trying that.
If your boot disk does not have an alternate for just such an occasion, I would strongly advise you to get one installed. Also, if possible get a backup boot device that is not used except for instances like this.
However, in this case, without the above resources, restoring from a CD may be your only recourse.