My iMac won't connect to the internet

My computer won't connect to the internet even though I know our internet is working fine and I am using the correct password. My phone and laptop connect just fine. Any ideas?

What version iMac, what version macOS, what ISP, what router, what connection, what have you tried, what happened, what messages? I have lots of ideas but we need a ton more information before any of them make any sense.

OS X Yosemite. The router is wireless but it has been connected for over a year up until a week ago. When I try to access our network and try to enter our password, it shakes as if the password is wrong. But it's right because I use the same password for my laptop and phone

If it shakes, the password is incorrect. No other explanation available for that result. So, type it in very carefully, check for caps lock, maybe reset the password, then reenter the new one on you laptop, phone and iMac.

My iMac is the 21.5 inch, late 2012 model, iMac 13,1

How do I change the password?

You'll have to look at the router instruction manual or user's guide for how to change the password for the WiFi network. It varies from router to router. But what changed last week? Things don't usually just stop working, something changed. You said,

The router is wireless but it has been connected for over a year up until a week ago.

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So what did you do a week ago?

I found sometimes I had to reboot my router when it was acting up, and then it smartened up.