Gen AI window disappeared

There is no specific floating AI window, there is the floating Contextual Task Bar which changes to offer options for many non-AI features as well as AI features. If the Contextual Task Bar is not visible, you can restore it the way Stephen Marsh suggested. All panels and bars are always accessible from the Window menu, so whenever any are missing, open them again from the Window menu. 

But also remember that in Photoshop, when you see a feature in the Contextual Task Bar, the context (right-click) menu, or the Properties panel, that isn’t the only place it is, because those are just shortcuts. You can always still get to them through the menus or tools. For example, even if the Contextual Task Bar is hidden:

• If you want Generative Fill when a selection is active, choose the command Edit > Generative Fill. 
• If you want Generate Image, choose the command Edit > Generate Image. 

One of the big reasons they make sure those are menu commands first is so you can assign keyboard shortcuts to them if you want.