error code -3403D

I received a software update from a well known site, one I use frequently and after initiating the update I got a prompt that the update was not compatible with my OS. I went to the apple support page and was told to run a diagnostic program (restart holding the "D" key. The result was a dozen screen and the following message: apple.com/support -3403D

My iMac appears to be frozen at this time. What to do?