Knock. Knock. Knock-knock. Knock.
Elsa ran to the door and even grabbed the handle, but she didn't open it. She had to remember what her father told her. Conceal, she thought. Don't feel. Don't let it show. It was just a few weeks after Anna's memory had been modified because of the incident in the ballroom.
She took her father's words to mean that she couldn't talk to Anna at all, at least until she can control her ice powers. As much as she wanted to, she couldn't open the door. She couldn't risk hurting Anna again.
Elsa heard her sister's voice outside the door. "Do you wanna build a snowman?"
I want to, but I can't, Elsa thought.
"Come on, let's go and play!"
I wish I could, Anna.
"I never see you anymore."
I miss you, too, Anna.
"Come out the door."
If I did, I'd hurt you again!
"It's like you've gone away!"
I'll always be here, even if you don't see me.
"We used to be best buddies, but now we're not."
I know, and I'm sorry.
"I wish you would tell me why!"
I can't, Anna.
"Do you wanna build a snowman?"
I might hurt you again!
"It doesn't have to be a snowman."
Elsa wished that she could tell her little sister the truth, but with her father's words in her mind, all she could think to say was, "Go away, Anna."
"Okay, bye."

A few years later, it had gotten to the point that Anna was knocking on her door at least a few times a weak, even thought she always got the same answer.
Knock. Knock. Knock-knock. Knock.
"Do you wanna build a snowman?"
I can't, Anna! I just can't! Elsa thought, wishing that her sister could understand.
"Or ride our bike around the halls?"
Please don't hurt yourself, Anna. You know how clumsy you are, she thought.
"I think some company is overdue."
I think so too, but I might hurt you again, and I can't let that happen.
"I've started talking to the pictures on the walls. Hang in there, Joan."
Elsa laughed silently to herself. I always knew that Joan of Arc was your favorite, she thought. Often the only thing getting her through the days of isolation was Anna's comments, even if Anna herself didn't know it.
"It gets a little lonely, all these empty rooms.
It's lonely in here, too.
"Just watching the hours tick by."
Elsa heard Anna run down the hall, so she stuck her head out of the door. She giggled silently to herself as she heard Anna clicking her tongue and saw her lying on the floor watching the pendulum of the grandfather clock swing back and forth and back and forth. Elsa had always loved her sister's antics but sighed and closed the door as she knew she couldn't spend time with her.

Years later, shortly after the death of the King and Queen of Arendelle, she heard the knock again for the first time in months, but it wasn't the usual cheerful rhythm.
Knock knock knock.
"Elsa?"
Unbeknownst to Anna, Elsa was already leaning up against the door, hoping and wishing for an excuse to open it, but she knew that she couldn't.
Yes, Anna? she thought.
"Please, I know you're in there. People are asking where you've been."
I can't come out, I'm sorry.
"They say have courage, and I'm trying to."
I know you can get through it, Anna. You can get through anything. You have more courage than anyone I've ever met. I wish I did.
"I'm right out here for you."
I know that, Anna.
"Just let me in."
I can't. You don't understand. I just can't.
"We only have each other. It's just you and me.
I know that, and I'm always here for you, even if I can't say anything.
"What are we gonna do?"
I don't know, Anna. I really don't know.
"Do you wanna build a snowman?"
Yes! Elsa thought. She wanted to scream and cry to her sister that yes, she wanted to be best friends again, and yes, she wanted to build a snowman. She looked around the room. Everything was snow and ice; everything was frozen, and just like her relationship with Anna, she didn't know how to fix it.
Elsa curled her head into her knees and started to cry. A single tear formed in her eye, and it turned to ice and shattered on the ground.