This might sound very unusual but one of the best way to overcome an unproductive day is simply just to REST and call it a day.
There are many reasons why people feel unproductive, but mostly the reason is just simply because they are tired physically or mentally. In a way, it's how the body tells you that you are overexerting and overthinking things and you need to chill.
Over fatigue, lack of sleep and stress can make you lose focus on work.. All of which can be solved by simply just RESTING. Go sleep, relax and have fun once in a while and by tomorrow, chances are you will be productive again.