The solenoid for the locking mechanism has given out or there may be a bad ground on the switch side. To get access to it you will need to remove the door panel.
Note: Be extremely careful some of the mechanisms have the windows attached to them and may have to be removed. This is not a typical job for the do-it-yourselfer the danger to the glass shattering is real and very dangerous and can result in serious personal injury. Have your vehicle serviced by a qualified service technician.
" What were dealing with is the battle between the future and the past, between the powers that were and the powers that have yet to be. " --- John Perry Barlow