What's confusing to me is that the second disk in this mirrored root pool shows c0t1d0s0 in one output and c0t1d0s8 is another output. I hope this is a typo. Otherwise, you need to find out why this device path changed. Its look like the primary disk is faulted. FMA will tell you if the disk is faulted and whether the second disk's device path changed. You will have to boot from media or an install server.
The following outputs should provide enough data to isolate a possible disk failure:
Are the disk paths consistent with with the ones before the reboot ?