If you have a 9 to 5 job, and are at work today, chances are your productivity peaked at about 9:15 AM.
I feel like I have this conversation with people a lot. Offices are not perfect machines. At some point, the innate absurdity of the cubicle kingdom sets in and you can only distract yourself enough to avoid a total breakdown in productivity.
Don't get me wrong, you're still working. But it's a sluggish kind of work, the kind that feels like you're in your own personal traffic jam, and you're looking for the next route where you can hit the gas with some comfortable sense of pace. You feel like "If I just had a coffee, I could get going" or "If I just try and dive into a task, I'll make it all the way to 5." But it just won't happen. Your spark will fizzle, motor will stall continuously. You're passed the threshold.
It's not for lack of motivation or lack of trying, just the fact that you are a human being and you are not meant to be doing this all day every day. Strictly speaking, your natural behavior as a human doesn't go much beyond walking upright and procreating. Offices aren't exactly engaging, dynamic environments. They don't produce stimuli to engage your senses, they aren't environments you can manipulate or improve upon.
They are off-white, gray mazes bathed in mind-numbing fluorescence. You can't be blamed for your lull. You can only endeavor to endure it while you squeeze out the last remnants bits of accomplishment from your work week.