Whenever I'm working in the develop module and I press F to get a photo full screen it won't let me press escape and return to the non-Full screen view. If I press esc it will return to the smaller view for a split second and then automatically go back to full screen. This just started tonight after successfully using Lr for months.
I'm on a MacBook Pro, 10.9.4 running Lr version 5.4. My photo library is on an external HD that is about 1/3 full.
The "F" shortcut cycles between the "Full Screen Preview" mode and whatever screen mode you are using. Try using "Shift-F" to cycle through the various modes until you get back the one you want. Alternatively, you can go to "Window" on the menu bar and look at the options under "Screen Mode".
I'm guessing you accidentally hit "Shift-F" and didn't realize it (I do it all the time).
Hope this helps.