My PC battery drained and went into sleep mode during a workout… when I plugged in the power and resumed, ZWIFT was showing Pause…so far so good. but upon restarting on the bike the Zwift display continued to show 0W and 0 for speed despite seeing correct readings on the launch page… so there was some glitch in the handoff from restart to actually riding.
I’m assuming the sleep mode is the main culprit here. In the future, I would advise against letting your computer go to sleep while riding 
Generally speaking, at some point the ANT+ signal might have been lost in the transition. Sometimes unplugging and re-plugging the dongle will help.