Assuming you don't have ADHD or something, and assuming you used to be able to, what other people are suggesting. But it takes practice, and it won't happen overnight.

I used to read a lot in school, then with college and work, I stopped having the time and energy and would only read short articles and newsblog sort of things. When I finally achieved some semblance of a work/life balance, I found it agonizing to try and read again.

You could also try starting with comics, manga, or graphic novels too. I felt part of my problem was "training" my brain to being able to read again. Also the usual be well rested, and not stressed nonsense too. Having a nice place to read and not having toworry about distractions too.