updated high sierra, now I have lost all my saved passwords in Safari and Chrome, what to do??

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", and then "Password" 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.

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.

For more info: https://discussions.apple.com/thread/8155947