Accidentally changed wifi security key?

[tumbleweed_biff]

Not at all.
You need to log into your router to fix this. If you don't have anything which is connecting to it now, you will need to get an ethernet cable and connect your laptop via that cable to the router. Then you will be able log into the router. If you don't know the password for the router, then what you will do is reset your router to default.

To do this, find the pin hole button on the back of the router, press it for 30 seconds. After the router reboots, it will be set to factory default. The default login information may be printed on the label on the bottom. If it isn't, just look up your router model and default password.

Step one:
Log into the router go to the admin page and change the password for the router.

Next go to the wireless page/tab. Turn on the WiFi and then turn on the security, choose WPA2 and enter a password. Make note if it is set to AES or TKIP. Log out of the router, disconnect your laptop, and connect by WiFi using the password you just set.