They are probably opening outside of your visible workspace - or under something else. If you can't find it, or you don't have a way of temporarily expanding your Windows desktop/resolution size enough to make it accessible again, then you'll need to go to General Preferences and reset to default. Snap a picture or commit you current set up to memory first, so you can later re-adjust everything back to your custom configuration once you have retrieved the missing component.

It happens to most of us sooner or later. Poser will let you drag things off the visible desktop, so it can occur quite accidentally. Once you do have everything repaired, consider saving it to a UI Memory Dot, so that next time you can just apply the UI Memory Dot to get it all back as it was.