Maybe I mis-spoke. At the top of the screen when reading a message, there is an arrow that takes you back to the Inbox that disappeared along with other buttons. I solved by problem by reinstalling the app.
I imagine you'd put the message view into "full screen" mode by accident - full screen mode has no "action bar" (the panel along the top which includes the back arrow) the account and folder names, etc.
When you do there is an icon, bottom left, to exit from this full screen mode. Might be easier than re-installing (if that's what it was).