Upon returning to the TARDIS, she had tried her best to comfort him, despite his protests that he was 'all right', until eventually she couldn't hide just how tired she was and the Doctor had sent her to her bedroom for some sleep.
Closing the door securely behind her, she collapsed onto her bed and closed her eyes, allowing herself to succumb to sleep. When she finally woke up, she felt a gasp escape her lips and she sat up, bolt upright, looking around her frantically. This was not her bedroom. Far from it, in fact.
"Oh, my God," she said, running an anxious hand through her hair, "Oh, my God."
Did he--? No. Had he just left her here? No. He wouldn't. He wouldn't. Well, then... where was he? Oh, God. Oh, God. Oh, God. Had-- Had something happened to him?