Thank you for asking this question in the support community. Unfortunately, there are multiple reasons you may not be able to connect via the GUI. First, if you "found the unit in your office," then most likely it doesn't have the factory default configuration on it, and therefore, the IP address may have been changed, or the web interface disabled, or the firewall enabled.
Here are instructions on how to access the web interface for reference: Accessing the Web Interface in AOS
Here are instructions on how to factory default the unit: Restoring an AOS Device to Factory Default
Please, do not hesitate to reply to this post with any additional questions or information. I will be happy to help in any way I can.
I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post and unmark it and select another in its place with the applicable buttons. If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.