Try asking your GP about a 'build up' type drink,such as Ensure plus.It comes in many flavours and fairly small bottles so its not too hard to drink.
I probably dont need to bang on about sensible,regular eating,but dont be too tempted by the cream cakes,belive me,they are difficult to stop!The reverse of dieting,aim to gain 2-3 lbs per week.
Add a bit of walking,housework or gardening and you will be soon fighting fit.Glad you are getting better and good luck :)
You need to mix your food groups as much as possible meat, carbs, fat, sugar this will put the weight on. If you cannot mix them on the same plate have the sweet and sugar side for desert straight after.
Other fattening foods you should eat:
Nuts esp macadama
suet pies and puddings
Pastry pies and puddings
Skimmed milk powder add to cereals and milk shakes anything
Protein and Iron for muscle buidling. Chicken, Turkey, Steak, Milk, fresh veg and fruit. Too much sugar is bad for teeth and may just give you a "spare tyre".
do you feel you are actually underweight now or is the weight loss currently just from the loss of muscle tone? Obviously you will need to take things easy but if you are allowed to do exercise just check what you can be doing and start a gentle build up to get stronger. Good luck and don't over do it!