In this case, the casualty was its X-band radio transmitter, the spacecraft's main line of communication with Earth. To save power, the fault protection system had reduced the rate of data transmission, and changed the X-band signal.
DSN reestablished the connection, and things seemed stable while the flight team began investigating the issue. But the next day, comms cut out completely.
NASA's suspicion is that the X-band transmitter had set off the fault protection system twice more, which would have caused the craft to turn it off completely. In its place, Voyager 1 would have switched to the S-band transmitter, which uses less power.
Unfortunately, it's also far fainter, and the crew feared that it could no longer be detected from this far away. After all, Voyager 1 hasn't used the S-band transmitter to talk to Earth since 1981, when it was obviously much, much closer to us.
Thankfully, DSN engineers were able to reconnect with this instrument, sending a command on October 22 that confirmed it's still working. The team doesn't want to turn the X-band transmitter back on before they can figure out what the problem is, but troubleshooting is ongoing.