Very likely you messed up the formatting of the weapons.csv file - did you edit it with a microsoft application like notepad or microsoft excel? Because if you did, that's probably where the problem came from. If you're on windows, I suggest using notepad++, it won't randomly change line-ending characters on you or decide that everything needs quotes or whatever silly formatting mistake microsoft's set on making this time.
Regardless, probably the best thing to do now is restore the file from backup - you did make a backup copy before trying to edit it, right? - And then try again once you've verified that you've got a functioning file to work from.
