Forum:PC Modding

Hey, whenever i try to mod my game in anyway and then load up the file it just starts up the introduction movie and has me pick a new character and start again, even when all i'm editing is by duplicating a single gun that is legit (one of the starting guns) anyone have any ideas what is happening and how to fix it? (oh and by the way i'm doing this for a little bit of fun just something to mess around with offline so please no bitching, i never play online)