After resetting default keychain, I cannot connect to Wi-Fi I used at that time
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.