Sound like you accidentally moved your document window off
screen. Try reducing its size using Window:Document Window
Size to a small value using, say like 90 x 90 and see if it
shows up. If so try moving to the center and then gradually
increasing its size.
Failing in that go to Edit:General Preferences and
in Docement Preferences and Interface Preferences
push the radio buttons for Launch to factory state.
Click OK, exit Poser and restart.