Focus, focus.... and it's gone

Have you ever experienced the feeling that you have to do a lot of work (either it is school of work stuff), but somehow you just get stuck. And nothing comes out of your hands even if you try to focus really hard.
I experience this definitely to often, but at moments where I lose my concentration I need to search for something to motivate me again! (Or at least something to motivate me to do it next time better!)