Treesa's right, more information would be good! How old is it? Are you keeping it warm enough etc etc...
I handreared one of my pigeons from around two days old last year, after I stupidly moved his nest and his parents abandoned him. I used Kaytee Exact parrot rearing formula made up fairly thick and I poured it into an empty and well washed out food colouring bottle. I cut a finger tip off a washing up glove and then cut a small hole in it and stretched it over the bottle top so the baby could put his beak into the hole in the glove as if he were putting his beak into his parents beak/crop. It worked pretty well but it's important to wipe off any mixture that gets onto the baby because it sets like concrete! LOL!
You will need to keep the baby warm if it's still not fully feathered. A reptile heat mat will do the job perfectly, just don't set it on too high a heat. Or a hot water bottle with a towel over makes a good temporary heat source. Also, keep an eye on the chicks crop, and only feed it when it's crop is empty or nearly empty or the food can go off inside the crop and make the baby ill.
Good luck with your baby! Mine made it to adulthood and he's now a great pet who comes outside with me and sits on my shoulder like a parrot!