iMac keeps asking for login

There was a power outage and when my iMac rebooted, it keeps asking for the password for various things - Cloudd, AddressBookSource, Messages Agent, etc. - even if I enter the password, it asks again in an endless loop. I tried to change the password in Keychain Access (as suggested by others) but it just locks up when I click the unlock button or select Change Password in the menu. I even went to Terminal and followed those steps but there isn't a file with a bunch of random numbers so that won't work.... I can't access the internet (wifi) so I'm kinda stuck!!