Was it adding two single digit numbers or proving Fermat's Last Theorem? One problem in 24 hours could be lightning fast!

Go sleep for at LEAST 8 hrs, shower with some nice-smelling soap, and eat a real meal. Then take a break while you're up and awake. It'll come back when you're rested, but you have to take care of yourself first.

take a step back and assess your life. look for stress, how you can improve your health, and emotional well being. when I get stressed even if it doesn't seem like a lot, but it is consistent, my ability to rationalize, do math or even remember words degrades. stress is super bad for you in large or consistent amounts, and it can take over and effect you in weird ways. lack of sleep is another one that causes fatigue, and once again varied reactions including confusion, and information retention difficulties. remove some stress, and aim for better sleep and you should see improvements, if not... see a doctor and they can give more insight and real, non-random dude on fml advice.

Burnout and depression are real. I strongly urge you to seek help.