I've never really had it. Ideas are constantly flying around my head like a snowstorm so my problem is the opposite - being overwhelmed rather than hitting a wall.
That said - I have struggled a few times when I've started thinking too much about what people will make of what I'm writing, while I'm actually writing it. That took a lot of the flow out of the writing and made it a trudge, and so not so good. In each case I've had to remember that when I'm writing a story it's for me alone - I want to know what happens and how. Once the first draft is complete I can start thinking about what other people will make of it.