Be forgiving of yourself. We all have off days where we are trying to squeeze a reasonable amount of productivity out of ourselves.
We all face those frustrating days where you have that block you just can’t brush off, but that is ok. We need to learn to be accepting of that not only for ourselves but others too.
The best thing to do when feeling like this is to talk, someone might have some tips to help you combat this or alternatively you may be given some different tasks to try and work on. Take a break, clear your head, and start again. The worst thing to do here is to try and deal with it yourself and spend the day aimlessly getting nothing done. Accept how you’re feeling and work on doing something about it!