How is this Separate External Preview Monitor connected?
- Computer Monitor via Video Card?
- NTSC Monitor via MATROX, BlackMagic, AJA?
- Mental Telepathy?
Window > Source Monitor
Sequence > Sequence Settings > Playback Settings:
- check/uncheck Desktop Video Display During Playback
- set External Device
Sounds like a Driver, Video Card, Connection, or Computer issue wherein the Computer lost communication with the Output.
Doesn't sound like a deal breaker if it's only happened once but I can see it quickly becoming frustrating if it becomes a reocurring thing.
Maybe double check your Video Card Drivers, Cables/Connections, and Premiere Pro settings to make sure everything is copasetic.