According to Nature, Hitomi might have perished due to a basic engineering error that started a sequence of unfortunate events. Apparently, one of the systems designed to keep it facing the right direction went on the fritz when Hitomi passed the South Atlantic Anomaly. That's a region over South America that exposes satellites to extra doses of radiation. After that particular system stopped working, the observatory started relying on a set of gyroscopes to face the right direction. The bad news was the gyroscopes weren't working properly, as well, so the space observatory spun out of control. Hitomi fired a thruster in an effort to right itself, but it fired in the wrong direction, causing the spacecraft to spin even faster.
In the end, Hitomi broke apart and lost its solar array paddles that were supposed to harness energy for a decade of data-gathering in space. Without those paddles, JAXA cannot restore the observatory's functions. The agency's biggest lost, however, was the satellite's impressively accurate X-ray calorimeter, which took three decades and three iterations to perfect. It would take $50 million and three to five years to rebuild it for another mission.
Despite Hitomi's untimely death, it was able to measure the speed of gas emitted by the Perseus galaxy cluster. That can't make up for JAXA's loss, but at least the satellite was still able to contribute to our quest to better understand the universe.
Part of JAXA's statement reads:
"JAXA expresses the deepest regret for the fact that we had to discontinue the operations of ASTRO-H and extends our most sincere apologies to everyone who has supported ASTRO-H believing in the excellent results ASTRO-H would bring, to all overseas and domestic partners including NASA, and to all foreign and Japanese astrophysicists who were planning to use the observational results from ASTRO-H for their studies."