2004 GMC Envoy I have to disconnect/reconnect battery in or

My Envoy won't start unless I disconnect and reconnect the battery. When I put it in park and remove the key, the "check engine soon" light still stays on, but all gages (and everything else) appear to be dead (until I disco/reco battery). Won't turn over, no clicks, nothing - dead. Sometimes the dinger will still "ding" (like the door's open with the key in the ignition), however the key's out & the doors closed. It's very odd. This just started yesterday & it won't start about 80% of the time now. I had a diagnostic test done (free) at AutoZone and no codes appeared so I'm at sqaure one. Any help would be appreciated as this is my only vehicle & I have a baby. Thank you so much if you can help me figure out what to get fixed!