Snowy display with diagonal lines on my iMac

Completed power supply repair on a late 2012 27” iMac and at same time upgraded to SSD. Got everything sealed up and when I turn on I do get the boot chime, but what normally would be the grey loading screen is a grey snowy static with diagonal lines. No problem with logic board prior to this repair. Any suggestions on what happened? Possible that display cable did not fully click in and attach properly? Or could I have damaged the display cable during the repair? Please help!