Go to your key bindings and assign primary weapon to left click, or reset all keys to default.
My weapon also will not fire... most of the time. There are times when it will, but the majority of the time it will not.
I did not test it in single player, but I did attempt to change my keybinding... as well as other keybindings and it will not let me change them. I restarted the game and it didn't fix it. At first I thought it was lag because I was watching Netflix at the same time. I am plugged in via ethernet, and there was no change when I turned off Netflix. I was also still able to effectively use my melee skill, and (at least when I played on xbox) that should not have worked well at all if I were truly lagging. I am going to attempt other fixes and test out single player, but I'm not so sure it will help.
