Why not just get a free estimate from a reputable contractor - ask their opinion about what happened and how to fix it. It's impossible to guess things from here and you don't want to start fixing it based on a guess only to find the problem is entirely something else.

There could be a couple of problems...1-The wood has swollen from moisture, The water has frozen and has frozen the door shut. Try a nearby window to see if it operates properly. If it does, chances are the house framing didn't shift and one of the 2 cases mentioned are likely. If you see paint peeling, it may be latex and has softened enough to perhaps act like a goo, offering a bit of resistance. You could also run a blow drier around the edges of the door for a bit. This will help to shrink the wood a bit and also help to melt any ice freezing the door to the frame.

Well, we had a contractor take a look and he suggested we get the opinion of a structural engineer. The only thing I can think of is that we had a recent bathroom remodel, with a new whirlpool bath sitting on some of the floor joists that abut the door frame. I see no cracks anywhere, the only thing I remember is that while the tub/floor of the new bathroom was level, the ceiling was not. We just disregarded it at the time, thinking it must be because of some early settling of the house (26 years ago).

I certainly hope it's NOT related to our recent remodeling project, but if it is, can reinforcing the structure be done without tearing down our new master bathroom??

Possibly. Do not expect complete strangers here to be able to seriously diagnose your house long distance. Call a professional and maybe do so quickly. It sounds as if that tub could be a problem and you don't want any dangerous issues to occur.

I don't think the OP is expecting anyone here to give an iron clad diagnosis. I think they are looking for some direction for an issue they have no experience with, that's what the forum is all about. Information is empowerment. I do feel, if an OP presents an issue, they should answer questions as they are presented by helpers and have the courtesy to provide follow ups to the people who have spent their time and free expertise or even just opinions. This really helps others in the future who read the thread and see what works and what doesn't.

Rhododummy...In most instances, tearing out the existing structure (i.e. remodeled bathroom) is not necessary. If indeed there needs to be some support work, the use of jacks and other means would be used. I think you should continue to look into having the problem analyzed and perhaps corrected since the integrity of any supporting structures is in question. I have seen these issues seem to go away once snow loads and temperatures are not an issue. Folks put the issue on a back burner and then it resurfaces later and sometimes is worse than before.

The reason I asked you to check a nearby window was to help determine the extent of any possible wall or even foundation movement. Not that it would be an exact diagnosis from long distance but, because there are certain characteristics peculiar to specific problems recognizable by an experienced person, of which there are many who participate in these forums. For instance, if brick is popping loose outside or the siding is bulging, you have a supporting wall issue that could be caused by water, termites/carpenter ants/wood borers, etc. If you are not seeing that, it's possible the foundation has shifted, depending on soil, seeping rain water, rising ground water, etc.

The problem could be (very common) due to water leaking through the thresh hold and deteriorating the support beneath. This would cause some shifting at the door. Although many folks don't like the look of a storm door covering their prized entry door, they serve several long term purposes with this being one of them.