I've just recently had this problem (It went on for a full month..and at the end my son wasn't eating ANYTHING unless it was M&C or PBJ.. god help us), so I brought it up at my sons doctor visit, and the doctor told me essentially.. Giving them Pbj or mac and cheese is fine, but only one per day, and essentially making it about the foods that are really good for them. If they don't want to eat it, put it away, and bring it back out next time they're hungry. She said to only give 4oz of milk with meals, No more than 16 oz of milk a day, No Juice, No snacks. It might seem like they'll never eat, but they won't starve themselves, and will eventually try the new foods, or old foods they used to eat regularly again.
After about 5 or 6 days of cranky hungry child only eating nibbles here or there, He finally broke down and started to eat what I gave him.. then it was like he'd never stopped.. he wants to eat CONSTANTLY now. It was torture to get to this point, but well worth it in the end, now that he's getting all the vitamins he should be again, and isn't so anti everything on his plate it opens up a lot more meal possibilities.
It helped us to get some of the Gerber Graduates Ravioli.. for some reason, they struck him as delicious again, after almost a year of having no interest in anything "baby" foodish.