They have backup transmitters and if everything fails (tower evacuation has happened) you talk to the overlying approach / tracon / center frequencies to check.

They have backup transmitters and if everything fails (tower evacuation has happened) you talk to the overlying approach / tracon / center frequencies to check. And then after that, land anyway and self announce positions for other traffic. It's basically the same as landing at an airport that doesn't have a tower. Happens every day at hundreds of airports.