Access to the Models dialog seems to have disappeared. This is usually available from the Models icon in a docked toolbar at the top of the application window.
The Models icon is usually accessed from the Primary Toolbar which is generally defined as the main default toolbar docked into the top of the application window. What has most likely occured would be that it has been dragged out or un-docked and exitted.
As the below video shows, the Primary Toolbar can be re-displayed by selecting it from the Tools menu and re-docking it into the application window. All of the Primary tools would then be accessible including the Models icon and pulldown option.
> Tools > Primary
If you find that the Primary toolbar is displaying ok but when you click on the Models icon that this dialog box is not displayed properly, similar to the below image, then it may be that the dialog has been accidentally shrunk down.
This can occur by clicking on the top or bottom edges of the Models dialog which can then modify the size of the window. The below video shows how you can resize this dialog by grabbing on the top or bottom edge of this window.
Toolboxes Disappear