If you're connected with a Windows 10 PC, Control Panel, Network & Sharing, left click your WiFi connection and then Wireless Properties. Select the Security tab and then the Show Characters box. You can also consult the router's web page. Select Details instead of Wireless Properties and go to the gateway web address (also your routers address) in a web browser. Here you should be ablt to see the WiFi password or change it. Another way around this problem is to press the router reset button to make it revert to factory settings. Then either look at the bottom of the router or the user manual for default settings.