Access router interface without wifi key?

Is it possible for a hacker to get into a router without knowing the wifi security key? My friend asked me to look at his wifi as he couldn't log on. He is adamant that he is still using the supplied key which is on a little label at the back of the router, but this now doesn't work. I re-set the router and tapped in the supplied key - instant connection. A few days later he lost the connection and the key doesn't work again.
Either someone is changing the password or the password is being corrupted. Or someone would have the key, but how?

Some routers are vulnerable using WPS

Set the router password and the wifi password to something else. Set wifi language to wpa2. Set the wifi namw to something else.
If it stops allowing login after these changes, its the router. If the password dowes not reset then someone was messing.

I would say someone had probably cracked the original key and by setting it back to the original it made it easy for them to change it again. Router really needs to be given its own secure key and WPA/WPA2 to increase the security. And the longer the key the better for security.

The router codes were admin admin when I re-set the router the first time and I didn't change them. So I re-set again and immediately set an access password for the router which is very strong. That seems to have done the trick, along with a new password for the network.. The original key was a sequence of letters, number, symbols and punctuation so how easy would that have been to crack?
I should add that my friend received an email fro his ISP telling him to choose a new password (for email) as his account had been compromised!

If it was a WEP key no matter what it would be easy to crack. If WPA or WPA2 then much harder...
Also WPS can be exploited sometimes... So best to turn that off too.

There is of course another way to crack a password.

Does your friend have any neighbours that in the past have been to the house but maybe not getting on so well with now? It is always possible (although perhaps not probable) that they took down the key off the router whilst at the house and then when there was a falling out started to randomdly change it.

This is by far the simplest way of "cracking" a password - physical access.

In the past I was staying at a relatives and they didn't know the WiFi key. Connected via cable, used the default admin/admin login and grabbed the key straight out of the router interface. They knew I was doing it so not a problem but it shows physical access can make this easy.

Loads of routers are vulnerable to a hack on port 32764 then there are the 50 million devices exposing uPnP on the internet. Lets not forget those makes of routers with their own individual vulnerability like Asus. It might be best to research your router model's vulnerabilities before you upgrade the firmware.