I have a 5 year male chihuaua that has just started to hide under tables and not eat...what's wrong?

Question

He was neutered 4 years ago and is usually very energetic.

Answer 1

he could be scared of something like have you changed anything? is someone new staying with you that he has never seen before? change scares some animals especially  small ones

Also he could be self conscious. A lot of people think dogs don't think like that but they do. my dog is anorexic and is a husky, I know another dog that wouldn't eat on tuesdays. those we caused by  the dogs being very self conscious. Hope that helped