After resetting default keychain, I cannot connect to Wi-Fi I used at that time I was going to install vim on Xcode; and I saw I've gotta reset default keychains (or something like this; system language is not set to English) and I did so.

The wireless access point I connected when I reset keychain stopped working; but other aps work as usual. The system says it cannot connect to Wi-Fi network and suggests to start diagnosis.

The diagnosis didn't give me any solution and I have no idea to solve this. Other computers successfully work with that Wi-Fi though. How can I get my computer connected to Wi-Fi?
When you reset your keychain you are actually discarding the saved password for websites and wifi networks. So, unless you are able to retrieve the old keychain in the Keychain app, I am attains you will need to enter those passwords again. 

You can also check if you have enabled the option iCloud Keychain in Preferences > iCloud, in case you have used it before. It stores passwords in iCloud. So if you have disabled this inadvertently, maybe you can make it work by reenabling that feature.