I step away from my desk and do as many pushups as I can. Break a sweat. Get the blood flowing and release some endorphins into the body.
I'll also write something completely different that has nothing to do with the scene I'm working on. Something funny, or dark. One challenge I sometimes do in my workshops is to "write the saddest sentence you can, in 60 seconds." Making yourself write short bursts with simple writing prompts will often be enough to reignite the writing you need to do with the scene you're stuck on. Keep a list of those simple writing prompts handy and practice them when you're NOT in writer's block mode, too, so when you are stuck they are familiar. There are lots of prompts and gimmicks to get the writing flowing--whole Pinterest boards and websites are dedicated to them.
(The best thing I've found, is that writing frequently and regularly builds endurance and gets the right synapses in your brain firing. That's more preventative of writers block, but I think it's worth mentioning here.)