Well, you could access the password and change it back, or change it to whatever else you like? (Assuming you mean the wireless password?)
You would need to login to your router admin interface to do this, via a computer which is connected to it.
This details of this procedure vary between router models. Most require you to type in an address such as http://192.168.0.1 then enter username and password.
I'm guessing that if the lodger has changed the wireless password (aka network key), the router username and password is still at the manufacturer's default settings. Have a look at the router itself to see if the login information is on there.
If you do login successfully, change that router login password and keep a note of it! That way the lodger can't change the network key. But don't lose the note .
If you have problems you need the router make and model number to help track down more info.
Thank you for your replies. I found the password but I've somehow changed something in properties , the security type, I've changed it back but now just have a Red Cross next to the wireless connection name. It says the settings saved on this computer for the network do not match the requirements of the network .
Help! I have no connection at all. What have I done and how do I input it right. I have done a restore but nothing happened .
Try going to Control Panel > Network and Sharing centre (or right click your network icon on the bottom right of the screen, and go via that route).
When you're in the Network and Sharing centre, on the left hand side, look for 'manage wireless networks'. Find your connection there, and click on it. You should see a dialogue box with 'connection' and 'security' tabs. Click the security tab and enter your new password. See if that fixes it.
If it doesn't, in the Network and Sharing Centre, try right-clicking your connection and remove it altogether.
Your network icon should show a balloon saying 'networks are available'. Select yours and re-enter your network key when asked. See if that works!
It's a while since I've done network troubleshooting, so am a bit rusty If you get stuck or it doesn't work, do post again.