The "options" sub-tab of the fiddler "Composer" tab lets you tear off the Composer into a floating window. Question is how to put it back? I lost the floating window and now fiddler's composer is gone. I didn't find anything in the menus to restore fiddler or its GUI to factory defaults. I tried uninstalling and reinstalling fiddler, no-go; Then uninstalling, grepping the registry and blowing away all keys that had anything to do with fiddler and reinstalling -- no-go. The only thing that worked was paving the machine -- (which I had to do anyway -- I didn't do that just to solve this fiddler problem!)
The floated Details View can be returned to the docked state by closing it.
In your case, it sounds like the view may have gone off-screen. On Windows 7 with Fiddler 2.4.1.1, a floated Details View shows up in the taskbar as a distinct Fiddler window. Hovering over the Fiddler icon on the taskbar, moving the cursor over the Details View preview, right-clicking the Details View preview, selecting Maximize, and then pressing Alt-F4 returns the view to its docked state.
Obviously, flattening your box and reinstalling Fiddler is also a solution. ;)