Changed battery on 97 ford expedition, won't start

You have activated the security system, and it sounds like it is in full shut down. Try the disarm procedure listed in the owners manual. If it doesn't work then your headed to the dealer for a complete system reset, make sure you take all the keys you have for that vehicle if you wind up going to the dealer. Now you might try this: from the inside manually lock all the doors except the drivers door, then with keys in hand exit the vehicle, close and with the key manually lock the drivers door, wait at least 3 minutes, then with the key unlock the drivers door and see if it will crank up and run, if not then you know where your headed.