Well, the way to access the filesystem is from your other boot; you should have already cloned to SD if you're going to do this.

Since (I assume) you don't have an SD with a bootable system on it, you'll pretty much have to reflash, or do serial console stuff.

If, OTOH, you have such an SD about (perhaps your update just hosed bootmenu as well as bringing the system down), you can hook up with the USB cable as though to flash, and run
to set the device booting off mmc. Then once you get in, you can reinstall your bootmenu.

(I just did that one to pull my N800 out of a non-bootable (and non-updated) internal system coupled with a freshly updated (and initfs-hosing) system on the SD; still have yet to recover the internal, but it was straightforward to kick things back up like this to regain the SD...)

Alternatively, if you were all set up with bootmenu + SD, you could have backed up just the initfs partition, in which case you can do a straight flash of that.That would be like
If you'd backed that up ahead of time, of course, you'd be restoring it, not asking what to do, but I figured I'd mention it for future reference. (I should have done this before, too; would have saved me the above hassle. )