Could be a "ghost" puncture: usually happens when its patched 99% correctly, a wee little bit of air leaks out so the tube can stay relatively hard for a few hours before you need to inflate again. They are a real pain in the ass because you never realize it until its too late. You suddenly flat out and when you check you can't find a physical puncture, and when you inflate the tube its almost impossible to hear such a small amount of air whizzing out. The solution is to simply patch it properly, except when you have multiple patches and have no idea which one was the most recent one....