Why is my iMac not connecting to wifi?

Please begin by reading and then following step by step the instructions in If your Mac isn't connecting to the internet over Wi-Fi - Apple Support. This usually solves most issues. If you continue to have a problem, I'd suggest calling AppleCare. You can find AppleCare's phone number in your area by clicking AppleCare contact Information

We cannot tell you unless you first provide us with some important information. Some questions are: did something change when this first began? Have you checked your settings in System Preferences > Network? Have you tried connecting via Ethernet? Have you tried turning your modem off and then on again to reset it. You might also do a computer restart in the Safe mode. That often fixes various problems.

Try shutting down the iMac and rebooting the router, then start up the iMac.

If this doesn’t connect, leave the iMac running and reboot the router

To reboot the router, turn it off (usually a switch on the back), count up to 10 and turn on again.