If your horse is fat, then you need to put it on diet. To get her to move you can try feeding a little bit of cereal grains to boost energy levels, but overall intake should be limited.
Find out the weight of the horse and feed it at 2% of body weight - turnout should be limited time or with a muzzle - however if horse is stabled for half the time and turned out the other half it can try to compensate what it didnt get before :P
Fat horses need low calorie diet - high quality forage for 6-8 weeks at 2% body weight, then lower the dose for the next 8 weeks if the horse is extremely fat to 1.5 %. Extremely fat horses need a lot more workout but no grain, oil etc. feed lots of fibre. Make sure she has enough vitamins and minerals for healthy life, but forage will give her the energy she needs and she will start burning the fat too.
A picture would help to try and evaluate how fat she really is, or try to get someone to use one of the body condition scoring systems to evaluate, how fat she is.
There can also be other reasons than just obesity that cause unwillingness to move - make sure the vet says she is OK to work harder and be on a diet.
Make sure if the horse is turned out on pasture that she doesnt have access to lush, sugary grass - the more she eats of lower calorie feed the better it will be :)
Has she been dewormed btw?