@1234.-_ 

See if you can log into the router's configuration webpage. (usually via http://tplinkwifi.net or http://tplinklogin.net or http://192.168.0.1 or http://192.168.1.1)

By default the username and password should both be "admin".

If you can successfully login, then click on "Wireless" -> "Wireless Security" and there you should see your existing Wi-Fi password in clear text.

In case you previously changed the login credentials for the configuration webpage and have forgotten those as well, then the only way to re-configure the router would be after performing a Factory Reset via the router's reset button. (long press > 5 seconds)