(UPDATE: OK It's back on again, but TEPCO still doesn't know what happened, according to their tweet at about 1:10PM their time on July 22. Looks like two power lines out of three that feed the plant went off. The contaminated water treatment system is still down.)
The contaminated water treatment system, which stopped on July 21 (again) when a worker shut off the water gauge of the tank that stores the treated water (what a strange design, shutting off the water gauge shuts down the entire system..), continues to stop because power is off at the plant.
According to NHK Japanese (9:16AM JST 7/22/2011) and Yomiuri Shinbun (11:05AM JST 7/22/2011):
Power to Reactor 3 and 4 was interrupted at about 7:10AM, due to the sudden power surge that tripped the circuit breaker at the temporary transformer. The cause for the sudden power surge is unknown.
The cooling of the Spent Fuel Pool of Reactor 3 has stopped. No discernible change in the pool water temperature so far (30 degrees Celsius).
Monitoring instruments for the reactor pressure are down.
The headquarter building at Fukushima I Nuke Plant is operating on a backup generator.
The cooling of the reactors and nitrogen gas injection continue uninterrupted, using the other power line.