The FAA timeline shows that NW188 was out of contact with ATC for 51 minutes, from 19:24:16 CDT to approximately 20:14 CDT.
It also makes clear that NW188 last contacted Denver center 91 minutes prior to 20:14 CDT, or 18:43 CDT, which means they flew in Denver Center's airspace for 41 minutes without Denver trying to contact them either.
They missed at least 2 radio calls from Denver Center.
NW Dispatch made 8 attempts to contact them, beginning @ 19:32 CDT (42 minutes before NW188 contacted Minneapolis Center).
At first I thought that NORAD responded in a timely matter, but it looks like now that they were very very slow to respond.