You can try using download mode to reinstall the software if the volume down button isn't working. To get into download mode make sure the phone is powered off, plug a cable into the computer or laptop port and while holding volume up plug the other end of the cable into your phone Keep holding volume up until you see download mode. If this worked you should be able to reinstall the firmware on your phone.
The below tool is LG Bridge tool and is used to reinstall firmware if needed, install the appropriate LG bridge for whatever system you're on whether it be Mac or Windows and use it to install your firmware.
[link|http://tool.lime.gdms.lge.com/dn/downloader.dev?fileKey=PWZO7A26B9UW|LG Bridge for Windows OS|new_window=true]