I am sorry about your bird.
First, check her for egg binding. Put on a latex glove with some KY Jelly or Vaseline on your index finger and insert it into her vent straight back. If there is an egg there, it will be right there within an inch or so. If it is 2 inches or more back, she is not egg bound and the egg is still safe in the uterus. If she does have a stuck egg, re lube up your finger and get this lubricant into her vent around the egg. Sometimes this is enough to get that egg to move. If she doesn't move the egg within 15 mins, bring her inside the house and fill up the sink or a basin with warm, not hot, not cool either, but warm water. You are going to soak her lower half in this warm water. The warmth should relax the abdomen muscles and allow her to push the egg out. You may need to hold her in the water, of she may sink down in and relax. Soak her for 15 mins. While she is soaking, get some calcium into her. Either some sort of human calcium or even Tums will work. Break them into pieces. If she won't readily eat them, stuff pieces in raisins and she should eat them. This calcium will help with contractions to move the egg out.
After 15 mins remove her from the water and dry her off with a blow drier. Then let her rest in the house for about an hour. The egg should pass within this time. Make sure she is able to eat and drink before putting her back outside with the others. And she needs to be dry as well.
If she is not egg bound, has she been wormed recently? Worms will impact the intestinal tract and not allow solids to pass. You will see lots of diarrhea, thin watery stools. If you have not wormed her or your flock, worm as soon as possible!!
Good luck and keep us posted!! :-)