Do you have a taskbar? Did they disappear from there too?
I run KDE plasma desktop on my laptop, and my home and work monitors have different geometries.
On display attach/detach, the desktop moves windows around rather intelligently but sometimes the geometry messes up and the window may end up atbsome ridiculous coordinates outside display areas.
On a desktop computer with multi monitors, the displays are loaded one after another at boot or resume from display-off mode, so the same geometric bug can still trigger.