Are you trolling or drunk/high?
It's happened to me before......You study the problem so long that your brain gets frozen and you say the heck with it and give it up. Then later in the morning the answer just pops into your head.
Kinda like trying to remember somebody's last name. I've been trying to remember the name of an old Detroit Tiger baseball player this morning but I just can't get it. I know later it will just pop into my head without even thinking about....