Model preview window not showing

When I douple click on my model then the preview windows only appear at the taskbar, but I when I click on it it just doesn’t showing for me. Yesterday it worked, I don’t know what happened. Can help me somebody ?

I found the “Reset Layout…” button in the window tag and it now works again.
Sorry for posting.

Thanks for posting! This issue has been plaguing me. Didn’t know about the reset layout.