IPhone 5 not letting me text?

I really need help here. My phone just stopped letting me text my girlfriend. Like I can Facebook, kik and all that to send messages, but it won't let me text her. It lets me text everyone else. She can text me and I get the messages, but when I reply, my phone tells me I sent it, but she doesn't get them… What's going on here and how can I fix it? I can't just keep using Facebook and kik because it uses up all my data!

Reset the phone?

if that don't work gotta reset the whole phone
make sure you hook it up to itunes before you factory reset.

Is she using iMessage or SMS? If you both have iPhones, they will try to communicate with iMessage if you are signed in. If either phone is not from Apple, they'll be able to communicate only via SMS over the cell network. In the former case, make sure that both of you are properly signed in to iMessage.

Not good