Restoring boot-block on Solaris

# cd /usr/platform/`uname -i`/lib/fs/ufs
# installboot bootblk /dev/rdsk/c0t0d0s0

If that's not successful, then do:

ok prompt
ok> probe scsi

If the disks are not seen yet, do:

ok> reset-all
ok> probe-scsi-all
ok> boot cdrom -s
Install boot block as above.
ok> boot cdrom -sw
# cd /
# fsck /dev/dsk/c0t3d0s0
# mount /dev/dsk/c0t3d0s0 /a
# cp /platform/`uname -i`/ufsboot /a/platform/`uname -i`/ufsboot
# cd /
# umount /a
# /usr/sbin/installboot /usr/platform/`uname -i`/lib/fs/ufs/bootblk \
  /dev/rdsk/c0t3d0s0
# halt