I quit MxO before it went to SOE and recently tried to start playing again but inputting my old code from the box is giving me an error saying it's an invalid code. Is this because I already registered it when I first made my account? And is there any way to get my old characters and stuff back? Or did I miss out on that and I have to start over again and get a new box?

Try going through SOE support/customer service.  They found my old account and I didnt even have my box.  I did have to give them as much info as I could regarding the old account.  If you paid by credit card and have the info still then that will help out also.