Routers have several passwords:
1) The configuration password.
2) The WiFi pass phrase - for a dual or multi-band router or one that offers a guest network there may be multiple WiFi pass phrases.
3) The ISP connection password, which is mainly used with DSL type ISP connections.
If you forget the first one, and it is not set to the default (in the manual or possibly on the router's label) then you have to reset the router to its factory settings and set it up again.
The others can be read by logging in to the router's configuration settings and going to the correct part of the configuration details.
If you have to reset the router, then check the manual. Many routers have a small concealed button hidden behind a small hole on the router's connection panel; however, the procedure for performing a reset can vary and should be checked with the manual (user guide) for the router. Some routers use a different reset mechanism. I have one router that simply requires a normal Ethernet cable to be connected between its Ethernet WAN port and one of its four PC/LAN ports when the router has power applied.
If you have a WiFi connected Windows PC running a recent version of Windows, you can find the WiFi pass phrase from the PC if you access an administration account, as follows:
1) Make sure the PC is connected to the WiFi network.
2) Open network and Sharing Center.
3) Click on the wireless network in N&SC to open the WiFi Status window.
4) In this window, click on Wireless Properties to open the Wireless Network Properties Window.
5) In this latest Window, select the Security tab and tick the "Show characters" box.
I hope this helps.