My Canon T2i wouldn't power on. It did this a couple of years ago, and despite watching other KZread videos on tricks to get the camera to restart, the only thing that worked was getting a new SD memory card.
My T2i wouldn't power on this week, and I again tried every trick on every video to no solution. Cleaning off an SD card, and reformating it was the only thing that got it to power on. However, trying another card, would kill it. I was able to both recreate and solve this problem.
It comes down to files on the memory card. I got into the habit of working off of memory cards when doing photo editing, and saving files in various locations, including the directories that the original photos were in, and other directories, and locations. This unfortunately can prevent your camera from powering on, as it reads the SD card on boot.