Why Did My Canary Stop Singing?

We put the christmas tree by her cage and she
was traumatized

There has  been no changes in his environment

Could be sick. Bring him/her to a vet to check it out. Don't take chances, as you wouldn't know if he/she is very sick.

Try whistling and talk to you canary more often, a strong bond with the bird can improve the bird's behaviour and thus brings out the best of it.

Maybe they get traumatized when they're moved from a familiar surrounding, so they're going to quit singing until they get used to their new surroundings.

Once they're familiar, you aren't going to hear the end of their happiness if you keep them well fed and watered.