The nut that you mention is attached to the tie bar that screws into the main unit above the burner module

what i would say and this being of my own experience is that your tank has frozen and expanded the water thus braking the tie bar ! This could be troublesum as not only has the tie bar broken your tank has expaneded and will need to be pulled out if possible ( i managed to pull mine back out ) you will also need a new tie bar and nut set and maybe a new seal which are availabe on ebay or online

you may have split your non return valve too and these are also online. i managed to fix mine while still in the van but alot or knee work involved and would of been better to take out and fix as that is what ive done to a couple of others since and alot easier

hope some or if not all of this makes sence

and of coarse its the carver cascade two water heater your taking about