This was a problem that was killing me about 6 months ago. Basically what was happening is that either the new forms I was opening (popup, modal) were rendering beneath the top form or off-screen (not sure which, I think off-screen). When this happened there was no way to pull the window back onto the screen - I would have to restart the application.
My workaround has been to make sure the form auto-centers when it loads.
Has anyone else run in to this, and does anyone have an idea of how to actually fix it and not just work around it?
Thanks!