Cannot access iCloud on my Macbook Air

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

I can't be sure, but...

First, Safe Boot , (holding Shift key down at bootup), use Disk Utility from there to Repair Permissions, test if things work OK in Safe Mode.

Then move these files to the Desktop for now...

/Users/YourUserName/Library/Preferences/com.apple.finder.plist

/Users/YourUserName/Library/Preferences/com.apple.systempreferences.plist

/Users/YourUserName/Library/Preferences/com.apple.desktop.plist

/Users/YourUserName/Library/Preferences/com.apple.recentitems.plist

/Users/YourUserName/Library/Preferences/MobileMeAccounts.plist

/Users/YourUserName/Library/SyncedPreferences/com.apple.syncedpreferences.plist

Reboot & test.

PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive.

Not sure if these ones needs to be or not.

/Library/Preferences/SystemConfiguration/com.apple.accounts.exists.plist

I'd be sure to bckup before moving this one...

/Users/YourUserName/Library/Mail/V2/MailData/Accounts.plist

Does this show you logged ion already also? Can you logout there?

Sign in to iCloud - Apple