Take a progress picture.
When you go off your meal plan, you will likely experience water retention and bloating from consuming higher amounts of carbs and sodium. This can add upwards of 10-15lbs of water to your frame. Depending on how long you went off your meal plan, and how many excess calories you consumed will determine what portion if any of the weight gain is body fat vs water retention. For example, if you only went off your meal plan for a day or two it's unlikely that you gained body fat and especially not 10-15lbs.
My suggestion is to not get on the scale right before resuming your meal plan. The scale reading could cause you to panic. Instead, take a progress picture, stick to your meal plan for a week, then take another progress picture. After a few days of proper re-hydration, sweating, and planned carb intake the water weight will come right off. If you did in fact gain some body fat, the before and after progress picture should motivate you to recover any ground you lost.
Most people just don't drink enough water when it's not top of mind. If you go off your meal plan, you likely stop drinking enough water. Dehydration can cause fatigue, which will hinder your workout, and faux hunger (feelings of hunger when your body is actually thirsty). Re-hydrate yourself by drinking lots of water, teas with zero calorie sweeteners, etc. I personally use Lyteshow which is bottled electrolytes and helps re-hydrate my body quickly.
Plan out your next 1 or 2 days of meals.
Fail to prepare, and prepare to fail. When you resume a caloric deficit, you will be battling all sorts of things like hunger, fatigue, cravings etc. Take a few minutes to plan out your meals for the next day or so. Account for fatigue by adding items such as oatmeal, coffee, or energy drinks to your meal plan, and account for cravings by finding healthier alternatives that fit your macros. If you try to wing it instead, you may find you never get back on track and sacrifice your progress.
When you go off your diet you will be consuming more high carb, fat, and sodium foods. As a result, that water weight I mentioned above many hit you like a bag of bricks. It will take a few days of resuming your diet, and staying hydrated before your water weight to regulate. You can speed up this process by getting intense-sweat-inducing workouts in.
Prepare to feel hungry.
Our bodies are constantly adapting and adjusting to its current state. When you go off of your diet for an extended period of time, your body will readjust to the higher caloric intake. When you go back on your diet, you'll be forcing your body to readjust to the lower caloric intake, which will trigger you to feel hungry and may affect your energy levels. This is all very normal, and in fact is an effective strategy to address the affects of slowed progress when on a caloric deficit for an extended period of time. Prepare to feel hungry, it's your body's natural response.
Grab a pack of gum, a cup of coffee, and some leafy greens to add volume to your meals. Soon enough you will be back into the swing of things!