Pro Tools Losing Internal Sync

[edit machine]

Hey guys, anybody had this issue? Lately and seemingly randomly, pro tools will lose it's sync, even though it's set to internal sync, it then displays this message:

The SYNC video reference is not present. Although the system will still play, it will not properly align playback to a video frame edge until the video reference is restored.

I'm only used to seeing an error like that show up if I'm using an external sync source and it gets unpatched (usually on accident by a machine room person). In those cases I just re-patch and I'm good. But I'm not sure what could cause the sync I/O box to lose it's own internal sync...