If access port protection is enabled on the device, access port 0 is unavailable.
The only way to 'reopen/unlock' the device is to issue an ERASEALL command through the CTRL-AP access port, and then issue a reset through the CTRL-AP. This will erase the entire code flash and UICR area of the device, in addition to the entire RAM. This method of erasing is slower than performing an erase all with the NVMC since it also must erase all RAM, but if access port protection is enabled, it is the only way to unlock the device.