Unable to log in to iMac after entering username and password

iMac will not log in after entering the username and password. The spinning pinwheel cursor shows for a few seconds then the loading wheel keeps spinning but doesn't log in. Also the "sleep", "restart" and "shut down" buttons didn't work on the log in screen after I attempted to log in. I have tried several times today and haven't been successful. I hate to hold the power buttom on the back of the Mac to reboot.

My Mac was working perfectly fine prior to this. It is a 2010 iMac. I installed El Capitan the day before this problem arose.

How can I fix this issue?