Throw that frozen mud away. Long time ago I worked for a real tight ass GC, he lost heat in his storage bldg over the holidays and we got a brutal cold snap during the same time. Ended up having nearly a whole skid of all purpose and lite mud freeze solid, the boxes of mud were harder than a concrete blocks! Boss refused to pitch it, even though USG rep stated that it should be. First thing I can tell you is the frozen solid boxes of mud took over a week to thaw enough to where they could be mixed. We eventually used it in a tract home when it finally all thawed out (IN March), the stuff never did mix well and had alot of problems getting it to flow properly through tools. Worse yet it cracked terribly on the surface as it dried, we ended up going back in to the unit where it was used and basically sanding it all off and redoing the entire house. Luckily I was an employee then, so I got paid for doing it, but the boss ate the bill. I've never even considered using any mud that has frozen since then, hell I don't even save partial buckets at the end of a job. If we've got a half bucket mixed up that's left over, we either pitch it in the dumpster or leave it for the homeowner.