In civilian applications, it is not legal to lock doors to prevent people from leaving the building since that affects life safety. So the only lockdown you can do as a security system installer is to lock doors so people cannot get in. Local authorities having jurisdiction (LAHJ) in some cases can decide to allow a delayed exit from buildings and some government agencies do not have to follow the rules of LAHJs so they can stop exit from a building.
One of the fastest growing use for lock downs is for schools and universities. Well publicized events have made this area highly visible. Lock manufacturers have come up with many solutions to enabling remote locking to prevent access to key areas.
Complete PC-based lock systems are available from several manufacturers as discussed in the complete article at the link below.