I need help getting my dog crate trained....?

I have a 6 month old Lhasa Apso. He has been sleeping in his crate for the 3 months I've had him and has done well in his crate after adapting in the first week. However, in the past week, he has been crying and scratching in the middle of the night and I don't know why. Any ideas as to his change of behavior and how I can get him to be quiet during the night again...... Other info about him, he is mostly housebroken (occasional accidents), he is fixed, and he gets plenty of exercise (30 minute walks 2x a day)