People have different work rhythms and cycles.
Some write in bursts of activity, lasting day and night, followed by extended periods of exhaustion. Others like to do it in the evenings (*cough*), some write in the morning... others work strict office hours, writing from 09:00 to 17:00.
But almost all writers find themselves at some point "unable" to write, even when there is time and energy to spare. You have the opportunity to sit down and write, nobody's pointing a gun at your head, your family's not starving... and yet you do not write. Why?
The late Douglas Adams was "late" also while he lived - he was notorious for not keeping deadlines. What was he up to? I don't know.
And there are people like Stephen King or Isaac Asimov, truly obsessive-compulsive writers - you couldn't stop them if you tried. But even they must have had their dry spells.
You can call it what you like - writer's block, fear of the Blank Page, procrastination, creative drought, waiting for inspiration... doesn't matter. Either you write, or you don't. "Writer's Block" is just a word. (OK, two words.) I'm not going to give you or myself any excuses.
Nothing gives me an ulcer like those poseurs who say (nasal tone): "Well you know, I want to be a writer..."
Well, who's stopping you, then? Are you waiting for your Official Writing Permit? Is Seinfeld on tonight? Just ****ing do it!
This little rant is aimed at myself too. Been procrastinating for several weeks. And I hate myself for it.