Hi- I had a Rebel T5 and recently upgraded to T7i.... I upgraded because of many reasons but the main reason was that my T5 would seem to "not wake up" and refuse to take a picture with my Canon EFS 18-55mm lens. I bought the T7i and I am now having the same trouble. I had the Canon EFS 18-55mm lens on and went to take a picture of my birthday cake and it would not take the picture. I tried different setting and I would press down on the button and it would not shoot.
Could it be a problem with the lens because this is the same lens that I used with my T5. Now I am having the same problem and am wondering if since that is the commonality between them that maybe somehow the lens caused this problem. I do NOT have this problem with my Canon 75-300mm lens.
Take a look at the post by Canon Product Expert Jesse in this thread.
He describes how to resolve a very specific problem related to the AF motors in the lens being jammed. If you remove the lens from the camera, and you hear a faint rattle when you shake it, then it is likely that you have this problem.