I’ve never come across this annoyance over these many years using Sketchup with dual monitors: all the dialog boxes are inaccessible, presumably off-screen. The suggested remedy is to reset workspace. But how does one go about doing this when the Preferences diabox is also inaccessible. Is there a key-combination or shortcut that resets the workspace?
Go Window/Preferences then hit alt and space together, then m, tap any arrow, then without clicking anything move the mouse around until the Preferences dialog appears on your screen, click to place.
meh… I figured it out…
I closed Sketchup, unplugged one of the monitors, started Sketchup (all missing dialog boxes appeared over the model), then plugged the monitor in. Easy-peasy.