Amnesia

First, they tried Rainbow Dash's home. Twilight stayed on the ground and waited for them to return.

"Does any of this seem familiar?" Asked Rainbow Dash when they arrived.
"Hmm. I don't think so." Said Fluttershy.

Rainbow Dash continued to show Fluttershy around, in hope that she will remember something. Anything.
After ten minutes of looking around her house, Fluttershy still couldn't remember her.
However, Rainbow Dash did not give up hope. Next, she tried showing Fluttershy pictures of them together.

"Oh! I know that photo! I remember the day it was taken!" Fluttershy said when she saw the pictures.
"Really? Great! Are you remembering me?" Rainbow Dash said, hoping for the best.

Fluttershy stopped and thought for a minute, and then looked back at Rainbow Dash.

"I'm sorry." Fluttershy said to Rainbow Dash

Rainbow Dash didn't know what else to try. She had just given up hope.
The two pegasi returned to Twilight.

"Did it work? Did she get her memory back " Twilight asked, but then she realised the answer when she saw the expression on Rainbow Dash's face.
So, the three ponies went home.
Fluttershy kept trying to remember Rainbow Dash, Twilight tried to think of other ways to help her friend and Rainbow Dash just sat there. Hoping the problem would fix itself.