|Are you referring to actual router's login password, or the wifi "access key"?|
Default "wifi"password is often shown on the base of the router - on a sticker; along with the router's SSID
When you reset a router (by pressing the reset button - usually with a paper click in a wee small hole on the back - it will reset the router settings to as delivered. Any changes you made to any of its settings will be lost.
Wifi is often disabled on new routers; unless it was supplied by your isp.
If you connect your computer to the router using an ethernet/cat-5 cable then you can login to the router's home page/management page(s). Once there you navigate to wifi set up. There you can see if wifi is enabled, and also see the default wifi access key (frequently incorrectly referred to a "password") .