Yes. You can absolutely regain your body weight and strength.
The reason you may seem so much weaker since the surgery is because an event like this takes its toll on your body and it will need to recover from this.
I advise you to seek professional help to get your weight and strength back to a healthy level because the is no way for me to give personal advise from the other side of the world via a forum like this. What I can do though is give general tips.
1. Consistency: This point can't be overstressed. Being consistent in the things I'm about to list will be the key factor.
2. Diet: Having a healthy diet is going to be one of the most important things here. Your body will need to recover from the fact that you were underweight. This doesn't mean that you want to go eat fast food every day for the next 6 months though. It also doesn't mean you need to eat loads. Your body will need to get used to eating a lot of food again so try to steadily build this up over time, increase it step by step.
3. Strength training: Right, since you're eating more and more, your body is getting the fuel to recover. Once you feel like you are ready to start working out, do so. Do something you enjoy, if you rather do circuit training than regular fitness does that. Doing something you enjoy will help you be consistent with it and you will naturally want to push yourself to become better at it.
Though I might add, doing more bodybuilding/powerlifting kind of exercises will make you grow faster than most circuit programs. So if that is the goal, you might want to focus on that kind of training at first.
4. Go easy on the cardio: Even though doing cardio is a very good way to stay in shape, it's not something you need to consistently do when you want to gain weight. It could hinder you from making gains you are trying to achieve. Once you start getting to the point where you are at a healthy weight and you want to become even more physically fit, you can add in some cardio.
5. Mindset: Don't think this is going to happen overnight. Make a plan for at least 6 to 12 months. Gaining weight from working out goes relatively quick in the first 6 to 12 months of working out, but don't think you'll gain 60lbs in a year, this is highly unlikely.
6. Workout partner (optional): Having a workout partner isn't something you need, but it can help in a lot of ways. It can motivate you to push a little bit harder, he or she can help you with dietary questions. I can highly recommend you try to find someone that wants to go on this journey with you.
I hope it helps if you have any questions let me know. As for some background information, I too was very very thin (about 120lbs at 184cm, male) and I gained about 45lbs in the first 2 years of working out. And I didn't have a forum like this so I didn't even know what I was doing the first 8 - 10 months.
