I have the same problem happen to me this morning. This didn't help me but try holding the power and down button at the same time until the system recovery screen comes up.
1st. Try booting into safe mode. If that doesn't work try wipe partition cache. If that doesn't work try factory device restore. None of these worked for me btw.
This just happened to me for the 2nd time in the past 3 months on a 2nd phone. I wish Verizon didn't sell me on the LG brand, because these phones are NO Good. I know have to wait until Tuesday for a new phone. The Verizon Rep didn't even try anything when I went to the store. They ordered me a phone wihout trying anything. In the past 20 years I have never had issues like this. I'm thinking about switch carriers, so they can buy out my contract. Verizon had to come up with some type of resolution.