Power off the router. Unplug it from the wall. Wait for few minutes.
Power off the router. Wait a while.
Connect the router back to to the wall.
Power the router back on. Wait until all lights are lit properly. It will take a while.
Restart the computer.
Start up in Safe Mode.
This exclamation mark in the Airport icon means that WiFi is working but the internet connection is lost.
You have to power off the modem, and the router, both . Then power up the modem and let it start up completely, then start up the router and let it start up completely, sometimes you also have to turn Airport off on the computer then on again when both modem and router are completely started up.
Thank you both, but neither one worked.
I have a "guest" access with my router, and when I log in on that network, I can get to the Internet. Moreover, other devices that are connected are working fine. So, I don't think it is the router issue.
I am not getting an IP address, though - when I look at the network under "Advanced" setting, the IP address space is empty.