I think that it's probably human nature to want to be good at things all the time, whether that is to just feel good about yourself or to impress other people. It could be at home, at work, uni or at training / competition. Society tends to emphasise being good, at your best, top of your game, all the time...which sets expectations and creates pressure to perform. But this is just not realistic, or even possible.
Just like the tide ebbs and flows, so does our performance, even when you feel like you are primed to go. "It's just not my day", or "My body just didn't feel like it today" sounds like cop-out, but it's real. This can happen at training as well as in competition...those days when everything is a struggle. And what's worse, feeling in a funk can last more than just a day or two...things might feel like a struggle for extended periods of time. Times like these are not just physical, but can affect your emotions, also.
What do you do when this strikes on 'game day'? The key is getting 100% out of what you are capable of on that day. If you're only functioning at 60%, then try to get 100% out of that 60%. This is when your experience from training comes in. Because training is also practice for how to perform at your best when it counts, you will (no doubt) experience all range of physical feelings while training. What did you do? How did you feel? How did training go? One goal for training is to learn about your body and how to get the most from it when you really need to. Learn from training.
But sometimes being in a 'funk' is longer term. After all, training is not always fun and enjoyable, or just does not come easily like it once might have...not necessarily because you've had some down / off time, but because you simply can't be good all the time. It could be for a few days...or a few weeks or months...or years; Olympic champions peak once each 4 years...who cares how they perform other times!!
When you feel like you are running through mud, with concrete legs, don't fret. Don't necessarily push yourself to do what your body is not feeling like it wants to do. Seek out some coaching guidance, check that your health is OK, and be easy on yourself. Let your body go through the dip so that it can re-bound to a higher high. Just keep being consistent rather than letting it all go. Stay relaxed and roll with it, and you'll be back into routine, and form soon enough.