Summarize the following:
If you don't know your Wi-Fi password and the computer in question isn't connected to the Internet, Ethernet is the only way you'll be able to connect.  On a Mac, you'll most likely need to purchase an Ethernet to USB-C (or Thunderbolt 3) adapter before you can connect your Ethernet cable. If Ethernet isn't an option, you'll have to reset the router to its default settings. In order to access your router's page, you must know your router's IP address:   Windows — Open Start, click the Settings gear, click Network & Internet, click View your network properties, and view the address next to "Default gateway".  Mac — Open the Apple menu, click System Preferences, click Network, click Advanced, click the TCP/IP tab, and look for the number to the right of "Router:". Common router addresses include 192.168.0.1, 192.168.1.1, and 192.168.2.1, as well as 10.0.0.1 for Apple routers. On some routers, the IP address is found on the sticker on the side of the router. Open your preferred browser and enter your router's IP address into the browser's address bar. You can use literally any web browser to perform this step. After entering the correct address, you may be asked for the username and password; if so, log in with your router credentials. These are typically different credentials than you use to connect to the Wi-Fi.  The default username is usually admin, and the default password is usually admin, password, or blank. Most people change these after configuring the router, however, so if you can't remember the username and password you may need to reset the router. If you didn't change the default username and password, it should be printed in the router's documentation or on the router itself. Once you are logged into the router, find the "Wireless" or "Wi-Fi" section. This can usually be accessed by clicking a tab at the top of the page or finding it in the navigation menu.  Each router page interface will vary, so you may have to navigate through a couple of different menus. Your router's password may also be at the top of the main router page. On the "Wireless" page, you will see your wireless network name (SSID) as well as the security or encryption type (e.g., WEP, WPA, WPA2, or WPA/WPA2). Near the security options, you should see the "Passphrase" or "Password" field. This is your wireless password.

Summary:
Connect a computer to the router via Ethernet. Find your router's IP address. Go to your router's page. Log into the router page. Go to the "Wireless" section. Find the password.