Force debug in safe mode via USB?
The easiest way to recover your phone is to flash the original factory firmware on it.

After much searching I found a thread at XDA which explains how to do this for the LG Optimus series phones including the P509 (yours).

You can obtain the original firmware by following these directions (substituting P509 which you have for P500).

After you recover, remember that the first thing you should do after rooting a phone is to install a custom recovery (usually Clockworkmod Recovery) as this gives you access to tools to manage situations like this.