I have to agree with BayCityBabe here. YOU are the adult. Giving her the silent treatment is hurting you as much as it is hurting her.
I doubt she intended to hurt you by not giving a card. I admit, I forget to call people on their birthdays all the time...even my own parents! My mom is constantly reminding me to call people. She even had to remind me to call my dad on his birthday this year. I don't mean to forget these things, but I'm almost 20 and life just gets busy. Between school, work, and everything else, things very easily slip my mind.
You have to remember that giving someone the silent treatment, or acting cold towards them without explanation only drives them away. It's like punishing a dog an hour after it had an accident on the floor. The dog has no idea why you are punishing it, and this only upsets both you and the dog. People are much the same way.
Remember that she did come to say goodbye to you before she left for college. Going away to school is a HUGE and very scary thing for a teen...especially if they have never been away from home before. How do you think acting cold towards her made her feel before this big life change? That may very well have influenced her actions on Thanksgiving.
I think what needs to happen here is that you should be the bigger person and apologize to her (Even though you may not feel as if you did anything wrong). Explain to her how you feel and why you feel that way. She may not apologize right away, but you will know that you did the right thing. The most important thing is to listen to her. Let her know you are always there. She is moving very quickly towards an adult world and should be old enough to listen to you. However, talking with her may reveal more about her than you already know and it is very likely that there is more going on than you can see.
When she is home for break over the holidays, offer to go to lunch or invite her over for a special meal. Spend the afternoon talking. It took me a while to realize just how valuable the relationship with my grandparents really is. I really don't think she is being rude or disrespectful. She is treating you the way she feels you are treating her...and she has no idea why you are treating her that way. I think a lot of this issue just has a lot to do with a misunderstanding and it would truly be a shame for the relationship to suffer over something like that.
I do wish you the best of luck. I know how hard these kinds of situations can be...especially when emotions just get in the way! It's never good to let these things fester though. I can bet that talking about it with her will make you feel a hundred times better.