Studies have found writer's block to be a simpler problem—unhappiness—than the legends around it suggest. But there are different kinds of unhappiness, and it's the blockee's job to be honest about which one they're suffering from.
The first, more anxious group felt unmotivated because of excessive self-criticism—nothing they produced was good enough—even though their imaginative capacity remained relatively unimpaired. (That's not to say that their imaginations were unaffected: although they could still generate images, they tended to ruminate, replaying scenes over and over, unable to move on to something new.) The second, more socially hostile group was unmotivated because they didn't want their work compared to the work of others. (Not everyone was afraid of criticism; some writers said that they didn't want to be "object[s] of envy.") Although their daydreaming capacity was largely intact, they tended to use it to imagine future interactions with others. The third, apathetic group seemed the most creatively blocked. They couldn't daydream; they lacked originality; and they felt that the "rules" they were subjected to were too constrictive. Their motivation was also all but nonexistent. Finally, the fourth, angry and disappointed group tended to look for external motivation; they were driven by the need for attention and extrinsic reward. They were, Barrios and Singer found, more narcissistic—and that narcissism shaped their work as writers. They didn't want to share their mental imagery, preferring that it stay private.
I bet group 1 (self-critics) account for most, though. Turn off your inner editor—and if necessary, move to a medium (longhand, typewriter) that deprives you of editorial tools