My keychain passwords gone after upgraded to High Sierra.

Retrieve password from keychain

Open Keychain Access.

Finder > Applications > Utilities > Keychain Access

Click the lock icon and provide the user login password to authenticate.

Select "Login", under Keychains, and then "Password" under Categories in the sidebar.

Double click the entry under "Name" to select.

A small window will appear.

Select "Attributes”, if not already.

Checkmark the box beside "Show Password" to enable it.

In the dialog that appears, enter the login password and click “Allow”.

The password will appear in the box next to it.

Write it down. Lock the lock.

Next time when the system needs to use the keychain, box will appear asking for the login/keychain password.

Enter the password, and then click OK.

Passwords for admin name , Apple ID , iCloud , wireless network password .... are to be written down in safe place , once you upgraded to Mac OS high Sierra , you can open safari preferences and check the box of user name and password so that when ever you login it will automatically get up filled , see this article macOS Sierra: Manage passwords using keychains

And there is no way to restore passwords even through time machine as passwords are always encrypted ( for security reason ) , even the Mac user can't read them .

I had the same problem.

After upgrading from Sierra to High Sierra my Keychain was almost empty.

I lost 2400 entries.

So I imported a backup of my login keychain and I copied all my passwords to the new login Keychain. But I had to validate each line...2400 validations...

Then even if the passwords where now in my login keychain it was not working for the emails with mail.. Only one solution destroy and recreate each accounts...What a lost of time.

Thanks a lot High Sierra. Regarding all the problems I had with Sierra, I choose to wait 2 updates and 4 months before upgrading to High Sierra.. Not enough apparently. Do not upgrade to high Sierra, wait at least six month or a year after the release the time all the bugs are solved....

One option: from a backup created prior to upgrading. Read Keychain for Mac: Copy keychains to another Mac.