Hello Nick,
I have a bid sad news for you... If you have made mistake many times you are not able to do anything. You need to go to the store you bought it from and they will fix it for you.
The bad news is that you will lost some of your data, those which are on the memory card might not be lost.
I have restarted it many times but nothing changes.. What I have to do to be able to use my LG Nitro HD again?