The telescope's non-essential systems have been turned off - and all science observations are on hold.
Hubble has six gyroscopes, all of which were replaced by spacewalking astronauts during a servicing mission in May 2009.
Gyroscopes are needed to keep Hubble pointing in the right direction during its observations 340 miles (540km) above Earth. The failed gyro is the last of the older ones. The telescope needs three working gyroscopes to "ensure optimal efficiency", mission team members have written, and the failure brings that number down to two (if the "problematic" one that had been off can't be brought back online).
NASA originally designed Hubble with six internal gyroscopes, but these components have a limited lifespan.
But we still have to stare down the barrel of an uncomfortable truth: Hubble is wearing out, and that 2009 service mission was the last. But when the third one was powered up, it wasn't operating as it should be, so NASA Goddard engineers placed the telescope in safe mode while they try to figure out the problem.
The troublesome gyroscope leaves two functioning ones, Space.com reports. The telescope is getting older, and that means that parts are starting to fail, but scientists are working to bring back a third gyro or work with a contingency plan.