I see a lot of fixes here to bring back the productivity but those may only worsen the problem in the long run. Depending on e.g. Work/life balance lately what you (and your brain) really need is a break. Just accept it, take that break and it'll get better soon. If it doesn't then you could consider actively solving the problem in any of the ways other commenters have given.

Just keep in mind that you cannot keep sprinting forever.