This is just a little one shot determining what could have happened if Elsa did open the door whilst Anna knocked all those years, It's nothing much but here it is...
"Do you want to build a Snowman?"
Elsa's stomach clenched as she heard the familiar voice of her sister, echoing through the key hole. She hesitated for a moment, leaving the room in an eerie silence. So many times she had rejected or ignored her sisters pleas to play. It broke her heart to know that Anna was at the other side of the door, expectant and hopeful of her appearance and completely oblivious to what had happened.
"Go away Anna." She eventually called back, resentful.
"Ok, bye." she heard Anna sadly say, before she heard her footsteps slowly walking away.
A wave of guilt washed over Elsa at the tone of disappointment in Anna's voice, even though Anna knew by now Elsa would never come out of her room to play, she still kept her hopes up and was always optimistic each time she'd knock on Elsa's door (which now was too many times for Elsa to count)
Elsa too was desperate to leave the confinement of her room and rekindle her friendship with Anna, but she was terrified of the outcome. What if she accidentally struck her again? In the heart? The old troll had confirmed that the heart was not so easily changed, so she shouldn't risk that possibility.
"Ok, bye." Anna's voice flooded through her thoughts, causing an overwhelming feeling of guilt flourish over her.
She gave out a loud sigh, I suppose, if I'm extremely careful, a few moments won't hurt anyone... She thought to herself, pulling at her cyan gloves tightly.
She slowly approached the door and turned the handle. Poking her head round the barely open door she looked down the deserted corridor.
She slipped out of her room and crept round the corner coming face to face with Anna's, and once hers, bedroom door.
She stroked the white wood and pressed her ear against the cold surface. She could hear muffled sobs and sniffles coming from inside the room.
Elsa's face twisted into a sad frown. She nervously and quietly tapped on the door.
"W -who is it?" Anna hiccupped, blowing her nose.
"Me." Elsa whispered through the keyhole barely audible.
"Elsa?" Anna gasped, her face lighting up as she ran across the room to reveal her elder sister at the other side of the door.
Elsa smiled as Anna's face brightened at the sight of her.
"Oh Elsa I've missed you so much!" Anna cried, attempting to wrap her arms around her sister in a friendly hug, but Elsa stepped away, holding her gloved hands out to stop Anna.
Anna looked up at Elsa in confusion, "why won't you let me-"
"I can't." Elsa cut in abruptly, folding her arms and sighing.
"But why?" Anna asked both curiosity and sadness in her tone.
"Because I just can't!" Elsa repeated in a more harsher tone. Suddenly the temperature dropped and a trail of ice formed around the frame of Anna's door and along the walls.
"W-what's going on?" Anna shivered, looking around in shock as icicles began to grow across the ceiling.
Instant worry flew over Elsa, 'I know I shouldn't have left!' She thought. Panicking she began slowly backing away from Anna.
"I'm...I'm so sorry Anna!" She stammered, attempting to round the corner before a sharp block of cold ice formed under her feet and tripped her up.
Elsa fell to floor with a loud thud and Anna stood in her door way shivering. Looking at Elsa in horror she cried out desperately for her parents as the hallway soon became filled with deep powdery snow.
"Elsa!?" The king bellowed, helping Elsa up in her drowsy state.
"Anna are you alright sweetheart!? What did she do?" The queen rushed to Anna's aid, her hand instantly flowing to her forehead.
"Anna your freezing cold! What did she do!" The queen shook Anna's shoulders as she seemed to be in a trance staring at Elsa.
"Anna!" The queen cried out forcing Anna to snap out of her trance.
"Nothing." Anna simply said. Her mother then held her close as she watched her father guide Elsa back to her room yet again.
Anna knew Exactly what would happen, Elsa would go back into hiding in her room and never come and play...but now she needed an explanation for the mysterious ice and snow. Had Elsa done it?