1.) You can try pressing and holding volume down and the power button at the same time to see if you can access recovery mode. There is an option for a hard reset there as well. If you're unrooted, it can be hard reset (factory reset) at that location. If you ARE rooted, it will not actually perform the reset but instead it'll launch you into your custom recovery where you can perform a nandroid restore.
2.) In lieu of the recovery option, you can also try pressing and holding the volume up button and while still pressing it, plug in your usb cable from a PC into the phone. This will activate download mode. If you can get that to work, you can reflash the stock firmware by way of the LG Flash Tools software.
If you can access neither of these, you are looking at a warranty claim.