Food issue 2.5 year old

Hi, hoping you can help me with this issue, I have 2 kids and the eldest (2.5 y o) has in the last couple of months starting refusing most foods. She used to be a good eater, and I worked hard to expose her to variety so that she wouldn't become a picky eater, and she used to eat anything you put in front of her. Now I'm struggling to get one piece of a toast A DAY into her! I've tried all the tricks, made food look appealing, ate it in front of her, allowed her to feed herself, got her to help me prepare food, etc... And she's started losing weight, so I'm very distraught, what do I do and who do I take her to? :(