Lafayette commemorative halves were issued in 1900. The only commemorative half dollar with that dual date was struck to honor the battle of Lexington and Concord. Please see the Related Question for more information.
If it's dated 1900, then it's a Morgan silver dollar. It's worth at least $20 or so in worn condition, and closer to $40 in uncirculated condition. If you have a 1900-S Morgan, the value is a bit higher.