When focus is given to the other Panels, and especially the Program Monitor Panel, perhaps the Source/Clip Monitor is being displayed behind it/them. In the Toolbar, under Window, do you see the Source/Clip Monitor checked? If not, when you do check it, does it appear?
Same general thought as above, but instead, does Resetting Workspace get that monitor back?
I would ask you to please check once if your display drivers are up to date or not.
Also check using Restore workspace as Bill mentioned.
Which format clips are you using?
In PRE 11, I don't think there's a way to reset the clip monitor -- under Window there is no Toolbar option, only Restore Workspace, Dual MonitorWorkspace, plus options for "Project Assets", "Add Media" etc.
And, clicking on "Restore Workspace" doesn't work. neither does right clicking on clip in Project Assets window and selecting "open in Clip monitor".
1.Yes, video drivers are up to date (NVIDIA GeForce 240 driver dated 10-2-2012)
2. Restore workspace doesn't bring back clip monitor
3. Clips are AVCHD from Canon Vixia G10 video camera -- project preset is HDV 1080i