I think the first step is to realize that you aren't a machine, that it's ok to not be productive.

You can also reflect and see if there are any emotions getting in the way of doing the task. Are you worried about something? Feeling overwhelmed by the task? Upset at you coworker/ or s.o.? Those kind of things. If so you probably need to work through those feelings.

Another thing to consider to consider is are you setting yourself up for success for the task. Do you have your needs meet? Food ? Rest? Exercise? Does music help? Etc. Do whatever helps you.

Finally a tactic that can sometimes work is to just do the bare minimum task and then to the next bare minimum.

Ie can you still at your desk? Can you turn on your computer? Sometimes doing it that way can help me feel less overwhelmed.