Default I rooted my phone and it says "security error" now

I rooted my lg optimus g (e970) and removed a bunch of the preinstalled apps that are supposed to be unremovable such as youtube and the at&t navigator, etc. I restated my phone but it says "security error" under the LG logo and turns off. This happens every time. I took it to at & t and they won't help me because they figured out I rooted it.