You can change layouts in Xmonad with mod+space or switch windows (in fullscreen) with mod+tab.
I encounter everyday the problem that these bindings stop working (I don't know what causes it). I'm able to change workspaces with mod+1 | 2 | ... | 9 and this problem occurs usually only in one workspace. I tried to solve it with moving all the windows from the workspace (where it stopped working) to another workspace but this didn't help.
I solve it with logging out and logging in. Is there some way to solve it without logging out?
Best Answer
It could be you mod+left mouse button draged a window in master -- this will float the window and cause the symptoms you described. (The layout actually changes, behind the window.)
mod+t should unfloat the window.