  2,1 code indicates system board failure. Make sure all connectors are plugged in properly, video card is fully seated and locked in place with the locking tab, and make sure you did not make any errors with the cable connectors and plugged them in incorrectly.  
I am not really knowledgeable to desktops, I just unplug and plug my GPU to clean dust then cannot start my desktop. Initially it just shutdown automatically, then I unplug power and reply GPU, after pressing start, nothing on monitor and flashing two yellow and 1 yellow. I check online and did some CMOS but does it work then I try to restart again, I can hear the sound but then just shutdown and power up again and again and again but still nothing showing. Could anyone help please, I am updating my resume and really need it to be done soon. Thanks a lot
Aurora R10, unplug and plug GPU, then cannot start