Contact your local SPCA or Humane Society to find a wildlife rescue organization immediately. The ASPCA site can help you locate one, possibly close to your location (see Resources section). A local veterinarian or wildlife rescue organization like Wildlife Rescue & Rehabilitation will also provide you with numbers. Although you may be able to lure the bird out by yourself, it is better to call them first, so they can send someone out who's trained in this area of bird rescue. If necessary, you can call and cancel if you're successful in the rescue effort before the experts arrive.
Figure out the bird's location by listening to the sounds it makes. If it seems like the bird might be moving around, you may be able to lure it out. If it stays in the same location, it's most likely stuck and will need assistance.
Determine the point of entrance after pinpointing the bird's location. If the bird is close to the entrance and moving, proceed to lure the bird out.
Shut doors to other areas of your house. In the room where the entrance hole is located, remove the screens and open all windows and doors to the outside. If you have blinds, close them over the area of the window that doesn't open to prevent the bird from flying into it. If there are no doors, hang a sheet over the doorway to prevent the bird from flying into other areas of your home.
Place a bright light by the hole to try and lure the bird out. Playing recorded bird songs or calls can also help entice the bird to exit from behind the wall. Don't use birdseed, though, as the bird won't be able to see it, and some birds have a terrible sense of smell. Keep quiet and see what happens.
Cover the point of entry after the bird has been removed. If applicable, use wire mesh or use drywall patch. The objective is to prevent birds from getting behind your wall again.