Sirius gave harry a mirror (which acts basically like a video call) to contact him whenever he needed him.

Sirius covered the mirror with some wrappers and gave it to Harry, telling him that this was a device to let him know if Snape ever gave him a bad time. He didn’t explain the true purpose of the mirror, since Molly Weasley was standing nearby and she would definitely not allow such artifacts to be taken by Harry inside school.

But, Sirius Black was thoughtful to write a note on the back of the mirror. The note says:

"This is a two-way mirror, I've got the other one of the pair. If you need to speak to me, just say my name into it; you'll appear in my mirror and I'll be able to talk in yours."

But during the moment Sirius gave the package containing this mirror, Harry decided not to use whatever was inside it. He thought he must not be the one to lure Sirius out of his house and get him into trouble. So he decided to “never” use it.

This is so stupid of Harry. I understand. He always thinks he is the one with the right mind. He thinks he will be the one who takes the right decision. That’s the thing which fails him.

He already made up his mind what Sirius was thinking. He thought the object was something that would make his dogfather come out of the place. And he thought Sirius would do that.

But Sirius was an adult. Maybe he was reckless and wanted to do what he wanted, but he did have the sense about what consequences his actions would lead. He knew that when he exposed himself in the public, what it would cost for the order. And he would do anything for the order.

You can also see how sensible he was, the way he looked after the children the night Arthur Weasley was attacked.

Although he did’nt grew up from the mentality of a twenty two year old, Sirius was still an adult. He knows what was right and what he should do.

But Harry refused to see that.

He did’nt tell Ron or Hermione about the package either (and if he told Hermione, she would have had the sense to use it when they needed to contact Sirius).

He totally forgot about it during the time it would have been highly useful, and went for the dumbest idea- using Umbridge’s fireplace to talk to Sirius and ask if he was okay.