Jijo, I think what you disabled was not the ability to boot from the NAND, but rather, the specific program that causes the Pogoplug to phone home, download a software update, and disable its SSH access.
If you want to boot into the original operating system on the NAND, but without running this program, just remove your USB stick and reboot. (It would be preferable if you shut the device down sanely before doing so.)
If you want to restore the disabled program, so that it runs when you boot your Pogoplug, you need to boot off the NAND, log in, and edit the /etc/init.d/rcS file, removing the comment character '#' before the line '/etc/init.d/hbmgr.sh start'.