Why isn't my IMessage not working for 1 contact?

My girlfriend and I both have an iPhone 4s. We're both updated with the latest iOS software (6.1.3). Yet for the past week it has not let me send her an iMessage. I also have an Ipod touch(my old iPhone 4) and an iPad (3rd generation), I have tried send her iMessages on all 3 devices, yet it still won't let me. I have already tried:
Turning all three on and off
Turning on and off the iMessage on all 3 devices
Signing out and signing back in on my iMessage for all 3 devices
Changing the contact name.

I have even tried turning off my phone off and then trying to send iMessage through my iPad yet it just says not delivered.
I NEED Help Please!

Sounds like trouble is on her end. Can you just call her or text her? Can she i'm you from her end. Worst case, she could have you blocked out, you know how women can be at times.

I try 2 txt my BFF but he's origanally from California but he move to North Carolina and I tried to text him on iMessage but it says not registerd but he is?!

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