Is she eating well? You should check with your vet if he is not eating well, he could be ill! If vet will say he is fine ask about how to bring his appetite back. We try to feed Tara one bowl a day and lots of treats. Train your yorkie and give him as a reward lots of yummy treats. Make it fun for him. I am sure he will happily join in if he is not ill. There's a great variety of treats, try them all I am sure you will find the ones he will enjoy. I hear yorkies love liver, fry some liver for him to jump start his appetite. Tara gets her on special days like birthdays or christmas and so on. There's also puppy food that is higher in fat that you could feed until he gains some weight.
Don't overexercise your yorkie, they are small and need only short walks, although my Tara is happy to walk even for two hours a day. But usually should not be out for more than 40mins -1 hour a day.
I hope this helps. Take care and make sure he is eating enough!
Let us know how she is getting on.
Sorry I just noticed it's a girl, she then!