Whenever that happens, I tend to focus on the development of my comic plot, writing-wise.
If I know I don't have it in me to draw in the moment, I go back to my plot and read over things (things that I haven't drawn yet).
Usually when I do this, I have a good part of my plot written out but not entirely finished. This allows me to go back to finish the rest of the plot. That way, I'm not putting my comic to the side entirely!
Another tip -- always try to do some gesture exercises when you have the chance.
Just quick 30 second, 1-2 minute, sketches that are rough, loose, and dynamic. Allows you to loosen up, get into the flow of another groove, and even improve your skills a bit more!