"No internet connection" showing on AirPort

I have had my MacBook Pro for about 3 years, and have never had a problem with the Airport before. Today I tried running diagnostics on my internet connection (my internet has issues sometimes) and suddenly there was a popup saying that my network settings had changed. Since then, my AirPort symbol (previously four wave-looking things) now have a small exclamation point over it, and won't go onto the internet. I assumed it was a problem with the internet, however it works fine on my iPhone and iPad. I looked the problem up in the forums, and many other people said that deleting some ".plst" files would solve the issue, however I can't find any of the files they are saying to delete on my system.

It is very incovenient because I can only go on the internet on my MacBook Pro when I have an ethernet cord attached to my laptop.

Would anybody know what problem I'm having, and how to fix it?

Thanks!