She did not have her wand at the time
Harry and Ron were planning on sneaking out to the Trophy room to have a wizard's duel with Draco. Hermione overhears this and warns them about being about at night (they could lose points!). The boys take no heed of her protests and decide to duel anyway.
“I couldn't help overhearing what you and Malfoy were saying —”
“Bet you could,” Ron muttered.
“— and you mustn't go wandering around the school at night, think of the points you'll lose Gryffindor if you're caught, and you're bound to be. It's really very selfish of you.”
(Philosopher's Stone, Chapter 9, The Midnight Duel).
Later that night, half an hour before the duel is supposed to begin -
“Half-past eleven,” Ron muttered at last, “we'd better go.”
They pulled on their bathrobes, picked up their wands, and crept across the tower room, down the spiral staircase, and into the Gryffindor common room. A few embers were still glowing in the fireplace, turning all the armchairs into hunched black shadows. They had almost reached the portrait hole when a voice spoke from the chair nearest them, “I can't believe you're going to do this, Harry.”
A lamp flickered on. It was Hermione Granger, wearing a pink bathrobe and a frown.
(Philosopher's Stone, Chapter 9, The Midnight Duel).
Hermione waits up to catch the boys in the act, and to try and persuade them to stay in their beds. She then follows the duo out past the portrait of the Fat Lady
“All right, but I warned you, you just remember what I said when you're on the train home tomorrow, you're so —”
But what they were, they didn't find out. Hermione had turned to the portrait of the Fat Lady to get back inside and found herself facing an empty painting. The Fat Lady had gone on a nighttime visit and Hermione was locked out of Gryffindor Tower.
“Now what am I going to do?” she asked shrilly.
(Philosopher's Stone, Chapter 9, The Midnight Duel).
With the Fat Lady gone they can no longer get back into the common room, even with a password -
It wasn't Mrs. Norris. It was Neville. He was curled up on the floor, fast asleep, but jerked suddenly awake as they crept nearer.
“Thank goodness you found me! I've been out here for hours, I couldn't remember the new password to get in to bed.”
“Keep your voice down, Neville. The password's ‘Pig snout' but it won't help you now, the Fat Lady's gone off somewhere.”
(Philosopher's Stone, Chapter 9, The Midnight Duel).
Now she is stuck outside of the Gryffindor common room, in a pink bathrobe, with (most likely) no suitable place to store a wand (and without a way to get her wand even if she wanted to). Keep in mind she wasn't planning on doing anything involving magic, only trying to stop Harry and Ron from going out and getting into trouble, so she didn't think a wand would have been necessary. She's not exactly dressed to go off adventuring and is more prepared to sleep than to use magic.
