(UPDATE: TEPCO started the backup system at 12:30PM, and nitrogen injection has started. Still no word about why the system stopped in the first place. Since TEPCO is incurious, don't hold your breath for the answer.)
(2nd UPDATE 4/4/2012: TEPCO monitors the system every 6 hours. A worker just happened to look at the monitor screen and noticed the amount of nitrogen injected showed zero. No one seems to know how long the system had stopped, and no one seem eager to find out.)
From Jiji Tsushin (4/4/2012, 12:24PM):
Nitrogen injection system for Reactors 1, 2, 3 at Fukushima I Nuke Plant stopped, TEPCO trying to restore the system to keep the hydrogen concentration low.
TEPCO announced on April 4 that the system that injects nitrogen gas into the Containment Vessels of Reactors 1, 2 and 3 had stopped. Nitrogen is being injected into the Containment Vessels to suppress the rise of hydrogen gas concentration , which could cause hydrogen explosions. According to TEPCO, it takes 30 hours before the hydrogen concentration reaches a dangerous level. The company is currently trying to restore the system.
According to TEPCO, before 11AM on April 4, a worker found the nitrogen injection system had stopped. The company is restoring the system, and trying to identify the cause.