During 2 years i began to loose weight and my face started to get thin. and now my cheeks are so thin. how can i gain weight (just face) again?

You can't gain...: ....Weight in just your face. If, however, you have been losing weight for two years unintentionally, then you need to be seen by a doctor for evaluation. That can be a bad sign. If you have been losing weight for two years on purpose, then you are seeing your face as it's supposed to be when you're thin.

Eat more meat and: Protein and do forced smile and chewing exercises. Hopefully these would help.Take care!