I have a two year old cocker spaniel who won't eat because we had to changed her food from what she was eating before?

Did you do a gradual change, taking about a week to change from one food to another.

Why did you have to change her food? Is she sick & has to have a special diet? Did you change just cause you wanted to? If the dog won't eat the new food, try another.

What was she eating & what did you change to? Why? Not enough information. If you can figure out how to edit to your question, please add more information. The more we know about what is going on with her the better your answers will be.

You can even post more information by answering to add more information.

The dog WILL eat when it gets hungry enough.

A dog can go for a week or more without food and suffer no ill effects as long as it has water 24/7/

AS said by others, change of food should be done gradually, but any healthy dog is not daft enough to starve it`s self to death, so don`t offer any food for 24 hours and then offer the new food.
If not eaten within 5 minutes, take it away and offer again in another 24 hours and so on until the dog is so hungry it WILL eat just about anything you offer it.

When you change a dogs diet, do it over time gradually switching from on to the other.

Maybe this may help
https://pets.webmd.com/dogs/guide/dog-not-eating-possible-causes-and-appetite-solutions