I want to gain weight

I don't know what's wrong with me but have no appetite for the last 8 months or more...only the mornings I have to deal with this. At first it was all day I wouldn't eat and didn't feel hungry but now its only in the morning and I have lost alot of weight over a short period of time...in a month or two I lost about 20 or more lbs and its because I sit all the day or sometimes just a little grocery shopping. I really want to gain the weight I've lost but I've been eating more but it does nothing. I want to gain about 25 lbs or more.