Two ways:
Easiest is to take out the disk you install on, mount it on another computer. Find out what you did wrong, and correct.
Less easily is to open it up, find where the pins are for the uart.
http://zyxel.nas-central.org/ showed where mine were, but seems down right now.
I've done both, for different reasons, on my NSA325. Neither are to hard. With the disk, you need to find out what you did wrong. usually follow the instructions in $this->bbcode_second_pass_code('', 'usb_key_func.sh.2') from
http://de5.mirror.archlinuxarm.org/os/nsa3x0/nsa320-alarm.tar.gz and just do it again. You won't be able to redo the u-boot environment, but can redo all the disk stuff.
The second you can see what errors you have, so easier to correct. You have to do if you have really messed up the u-boot environment. Otherwise, after finding the fault, you'll probably still have to pull the disk.