Filer-A was dead completely, "system console" showed nothing, "cf status" on a survived Filer-B showed only that it has taken over a partner. Interconnect link was down too.
Then I issued "system power off" on a dead filer's SP. Powered it on after 5 minutes.
Bingo!
All hardware (voltage) issues were auto-deasserted, ONTAP booted normally and now is ready for a giveback.
Now I'm sure that my system was hit by a 500 days uptime bug. Upgraded my SP's firmware to latest compatible version.
Thanks all for help.