There are lots of different types of writer's block. For instance, if you drank alcohol or took some kind of sedative, your brain isn't working properly yet and in fact it won't work until you stop panicking.
The other thing you can do is go for a walk of at least 20 minutes, ideally an hour. Walking seems to magically rev up your imagination - and no, the cycle trainer and the Pilates won't do it. You need to be walking somewhere, looking around you. Outside.
I find cities or towns much more inspiring than Nature - just saying.
So next time that happens, go for a walk.