Why does my son refuses to eat healthy food?

My son used to love steamed veggies and raw veggies such as cucumbers. Now for the past couple of weeks he is refusing, absolutely refuses to eat anything but cookies, sweetened yogurt... He simply does not touch his food. He will rather go to ber hungry than eat. What should I do? He is 2 1/2 years old.
Update: thanks so much for all the answers. It helps a lot.
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