freebsd stable - May 2012 - urgent system suddenly boot failed due to ZFS:i/o error on FreeBSD 8.2

hi,lists
  for some reasons I need restart system but it suddenly boot failed today.
here is the error:
ZFS: i/o error - all block copies unavailable
ZFS: can't read object set for dataset 16
Can't find root filesystem - giving up
ZFS:unexcepted object set type 0
ZFS:unexcepted object set type 0

FreeBSD/i386 boot
Default: zroot:/boot/kernel/kernel
boot:
ZFS:unexcepted object set type0

FreeBSD/i386 boot
Default: zroot:/boot/kernel/kernel
boot: _

any ideas to resolved it?
thanks in advance