My WiFi password got changed automatically

This morning, all of a sudden, my wifi password got changed without my knowledge. Obviously, the first thought would be if someone else changed, but that is not possible at all, as I was the only one using my WiFi.

I read on the internet that it could have been a glitch or a bug or my router does this once in a while to reset password. But I have been using this router for years and I have never encountered this issue before. That leads me to think if I have been hacked. I connected the router via ethernet cable and tried to change my password at routerlogin.net, but it says the admin creds are wrong. I don't remember changing it, It was the default password. But I couldn't get in. 

Tried to recover admin password, but the serial number invalid message pops up. No idea what to do. 

I pushed the reset button in my router and my wifi password worked. I'm not sure, because I read, on pressing the reset button, everything's reset, and so my wifi password should be the one on the router sticker and the admin creds must be reset to default. None of it happened. My WiFi password was set back to my current Wifi password which didn't work few minutes back, and the admin creds is still unknown.

Two things that I could have happened is:

1. My admin creds have been somehow compromised, which means they changed the wifi password, and they still have control.

2. Or it was indeed a glitch as they as, which on router reset got rectified and I forgot the admin creds which I changed.

Either way I need a way to change my admin creds. Any thoughts?