The Wi-Fi password can usually be found on a label on your router or on the information sheet that was provided with your router. This is a standard practice for most internet service providers. If the Wi-Fi password is not found then try to contact your internet service provider to see if they can provide you with the Wi-Fi password.
Your internet service provider may suggest that you manually reset the router to its factory defaults and follow the instructions in the routers manual on how to setup the Wi-Fi again. Please be aware that you should under no circumstance reset your router to factory defaults unless the provider of your router says it is okay.
If you no longer have the manual of your router you can probably find it on the internet. Start by locating the model name and number on the router itself and search the internet for the phrase "<insert model name number> manual". Please be aware that if you reset your router that all your connected devices may need to be reconnected to your Wi-Fi network again.