[Andrew WCY]

Like what others have said, don't worry about it! It happens to me all the time too. I could play okay just two days ago, but today I couldn't even read the notes properly no matter how hard I'd tried.

There's no point in getting frustrated over being suddenly unable to play at the level you could previously play at. When times like this come, give some easier files a shot or take a break for a day or two before playing again. From my experience, this might be an indicator that your skill's improving: It's akin to how you're too tired to do exercise for a few days after a strength training session, but you find yourself having better strength following many cycles of rest and training.

Keep playing and you'll gradually improve