in cold basement approximately 40-45 degrees, it was completely frozen about 3 lb. brisket. Discovered it 24 hours later. It was cold to touch, and completely thawed. Still in original vacuum packaging from store. Will this be safe to cook?
I'd be fine with it. Salt is a preservative, and it was still cold to the touch - not the same as if it had spent 24 hours in a 70 degree F environment, or a 40 degree C environment.
The default mantra for this site is to throw everything away. If you or someone you are feeding actually is immune compromised, this makes sense - if not, it's a rather extreme view, but that's the way the wind blows here, so that's the advice you can expect to see upvoted.