Zyxel

I have the following connected to my Zyxel Primary Router.

Wireless Adapter (Located upstairs)

Another router connected via LAN cable (Located in the lounge)

I've setup this router and everything along with all the other devices with their individual SSID broadcast names and password. Everything works fine for a few month then all of a sudden you can't connect to the primary Zxyel router's WiFi.

It indicates that the password is incorrect although it's the correct password that was entered. It's is the second time this occurred. What could be the possible problem here? I've tried the default router wifi password. It's like the password changes without anyone changing it. Strange thing is that the wireless adapter connects and pick up internet access as well as the secondary router in the lounge. It's just the Primary Zyxel router giving me issues. Could it be faulty?

Any help or suggestions would be appreciated.

Thanks in advance.

A