If you have a partial blockage on the main line, the toilet won't flush when there's no place for the waste to go. Overnight, it might drain down, and the first couple of flushes might work right. But, take a shower, or two, and if there's a blockage, it could back up enough to then prevent the toilet from flushing.
I'll keep this in mind and if it happens, I'll call the drain cleaning company referred to me by the plumber. Thank you.