For weight loss, your first culprit will be your diet, specifically calorie intake. Exercise is good for your health, and there's evidence that it allows to be healthy eating a wider variety of foods, but unless you're training for hours each day, you're better off looking at what you're eating and how much. I would bet that if you noted down everything that you ate or drank (including alcohol) for the space of a week, you'd be surprised at how much food you didn't even acknowledge eating. Snacks add up, and it's been estimated that most weight gain happens over holidays and weekends when you're not paying as much attention.
If you're more concerned about your inability to get back into exercise, as your second paragraph seems to indicate you're having trouble even starting, consider whether your expectations are set on your past performance rather than your current ones. Don't worry too much about how long you can run, or how much weight you can lift. Look at doing something of moderate intensity at least three times a week for about 20 minutes. Depending on your fitness level, this could be as easy as taking a brisk walk around the neighborhood, particularly if stairs and hills are involved. Aim to have an elevated heartbeat, but to keep yourself from breathing so heavily that you can't talk. You could go for more exercise, or more intensive exercise, but frankly, it's far too common for people to push themselves too hard and to quiet as a result. If you can't get out to exercise, then calisthenics such as pushups, pullups, dips, and just doing silly little things like hopping in place aren't a bad way to get your metabolism up.
Most importantly, when it comes to diet and exercise, don't try to be a saint. Without small indulgences (which you will note down in your log of food), you're never going to keep to a diet for more than a few months. And if you're not enjoying your form of exercise, you're not likely to keep to it in the long run. Find foods that you don't have to choke down and vary your exercise routine whenever it gets boring or feels too onerous.
Good luck! And be realistic about your goals. Sustainable weight loss tends to be at the 1-2 pound per week level for most people, and your weight will fluctuate a bit over the course of that week depending on what you eat and drink.