Short version: a flooding event in the Engineering spaces caused an electrical fault that scrammed the reactor. No reactor, no propulsion. Ship attempted to emergency blow, but water in the airlines froze solid and prevented that. No propulsion and negative buoyancy initiated an unrecoverable depth transient which exceded crush depth.
At the time, reactor plant procedures did not allow for a quick reactor startup to restore propulsion. This has been rectified such that restoring and maintaining propulsion is the #1 priority, even if it risks reactor safety.
I was on USS flasher SSN-613. Sister ship to Thresher, scheduled to be next out of the yards after Thresher. When it sank, navy made a bunch of changes in design that required retrofitting and therefore Flasher was delayed and other subs were completed first. One interesting thing added was a 3 inch emergency blow line, which bypassed the normal 1 inch ballast tank air line, because the 1 inch blow iced up on Thresher when they were trying to surface with no propulsion, and no more air could get into the tank to get more buoyancy. Sorry if that is a repeat of the article. Also had electric motor to the main shaft, and a ships propulsion motor (like an outboard) that could be lowered for emergency propulsion. Lots of changes resulted from Thresher. That class of submarine was originally called the Thresher class, but was called the Permit class after the next sub out of the yards.