I was trying to erase all of the personal content off of a 3G (it was jailbroken with 3.1.3 on it) so I can sell it. I went through the settings and chose "reset all settings and erase all content". Now it's stuck at the apple logo. Looking at google it appears that what I did actually wiped out the OS as well. How can I get 3.1.3 back on the phone?