"Coach, I've not done my session today, how should we adapt the rest of the week?"
There are three main reasons why I love my athletes asking this question:
They're not hiding. Rightly or wrongly, the majority of people feel some sort of shame or guilt when they don't do the session that is prescribed on a given day. Irrespective of the reason for not getting the session done, that guilt often leads to people 'hiding.' In other words, they go quiet, hoping that coach won't notice. Trust me, when athletes go quiet, coaches know there's a problem. By actively communicating with your coach, rather than going quiet, this brings the problem out into the open, and the pair of you can problem solve it together. This is where a coach is often at their most useful, when things are not going smoothly. Don't hide.
They're not playing catch up. When people hide, they also tend to go a bit rogue with their training plans, adding in little extra 10 minute sections to runs, running a little harder to make up for that missed session, trying to cram in the missed session to an already full rest of the week. We've all done it. I call this 'playing catch up' and it's unnecessary. Missing one session every so often will not kill you or destroy your training. If you are sick or overly tired, it will actually benefit you. Playing catch up on the other hand is stressful and increases the risk of injury as you are tinkering with a training plan that is designed to do specific things, through specific sessions and timings.
It shows trust and humility. Coaching relationships depend on many elements, but trust is amongst the most important. When an athlete asks a question like this, it shows far more than what appears on the surface. In my eyes, these types of question show trust and respect for the knowledge of the coach, placing trust in the coach' experience of dealing with similar situations. It also shows that the athlete is curious to learn and that they are self aware enough to know that they might not know the answers right now. All good news in my opinion.
So if you've missed a session, sure, it's not what was originally planned but that coaching knowledge of how best to instigate plan B is one of the main things you are paying for, now is the time to use it. Trust me, your coach will be far more grateful for the question and the honesty, than you hiding and going rogue with your training plan. You will both benefit as a result.